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  • Anton Anton anjo
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  1. Jul 17, 2017
  3. Feb 21, 2017
  5. Jan 16, 2017
  7. Jan 03, 2017
    • Stefan Hajnoczi's avatar
      aio: self-tune polling time · 82a41186
      Stefan Hajnoczi authored 
      
This patch is based on the algorithm for the kvm.ko halt_poll_ns
parameter in Linux.  The initial polling time is zero.

If the event loop is woken up within the maximum polling time it means
polling could be effective, so grow polling time.

If the event loop is woken up beyond the maximum polling time it means
polling is not effective, so shrink polling time.

If the event loop makes progress within the current polling time then
the sweet spot has been reached.

This algorithm adjusts the polling time so it can adapt to variations in
workloads.  The goal is to reach the sweet spot while also recognizing
when polling would hurt more than help.

Two new trace events, poll_grow and poll_shrink, are added for observing
polling time adjustment.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-13-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      82a41186
    • Stefan Hajnoczi's avatar
      aio: add .io_poll_begin/end() callbacks · 684e508c
      Stefan Hajnoczi authored 
      
The begin and end callbacks can be used to prepare for the polling loop
and clean up when polling stops.  Note that they may only be called once
for multiple aio_poll() calls if polling continues to succeed.  Once
polling fails the end callback is invoked before aio_poll() resumes file
descriptor monitoring.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-11-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      684e508c
    • Stefan Hajnoczi's avatar
      aio: add polling mode to AioContext · 4a1cba38
      Stefan Hajnoczi authored 
      
The AioContext event loop uses ppoll(2) or epoll_wait(2) to monitor file
descriptors or until a timer expires.  In cases like virtqueues, Linux
AIO, and ThreadPool it is technically possible to wait for events via
polling (i.e. continuously checking for events without blocking).

Polling can be faster than blocking syscalls because file descriptors,
the process scheduler, and system calls are bypassed.

The main disadvantage to polling is that it increases CPU utilization.
In classic polling configuration a full host CPU thread might run at
100% to respond to events as quickly as possible.  This patch implements
a timeout so we fall back to blocking syscalls if polling detects no
activity.  After the timeout no CPU cycles are wasted on polling until
the next event loop iteration.

The run_poll_handlers_begin() and run_poll_handlers_end() trace events
are added to aid performance analysis and troubleshooting.  If you need
to know whether polling mode is being used, trace these events to find
out.

Note that the AioContext is now re-acquired before disabling notify_me
in the non-polling case.  This makes the code cleaner since notify_me
was enabled outside the non-polling AioContext release region.  This
change is correct since it's safe to keep notify_me enabled longer
(disabling is an optimization) but potentially causes unnecessary
event_notifer_set() calls.  I think the chance of performance regression
is small here.

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-4-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      4a1cba38
    • Stefan Hajnoczi's avatar
      aio: add flag to skip fds to aio_dispatch() · 721671ad
      Stefan Hajnoczi authored 
      
Polling mode will not call ppoll(2)/epoll_wait(2).  Therefore we know
there are no fds ready and should avoid looping over fd handlers in
aio_dispatch().

Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-2-stefanha@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      721671ad
  9. Jul 18, 2016
  11. Feb 04, 2016
    • Peter Maydell's avatar
      all: Clean up includes · d38ea87a
      Peter Maydell authored 
      
Clean up includes so that osdep.h is included first and headers
which it implies are not included manually.

This commit was created with scripts/clean-includes.

Signed-off-by: default avatarPeter Maydell <peter.maydell@linaro.org>
Message-id: 1454089805-5470-16-git-send-email-peter.maydell@linaro.org
      d38ea87a
  13. Nov 09, 2015
  15. Oct 23, 2015
  17. Jul 22, 2015
    • Paolo Bonzini's avatar
      AioContext: optimize clearing the EventNotifier · 05e514b1
      Paolo Bonzini authored 
      
It is pretty rare for aio_notify to actually set the EventNotifier.  It
can happen with worker threads such as thread-pool.c's, but otherwise it
should never be set thanks to the ctx->notify_me optimization.  The
previous patch, unfortunately, added an unconditional call to
event_notifier_test_and_clear; now add a userspace fast path that
avoids the call.

Note that it is not possible to do the same with event_notifier_set;
it would break, as proved (again) by the included formal model.

This patch survived over 3000 reboots on aarch64 KVM.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Tested-by: default avatarRichard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      05e514b1
    • Paolo Bonzini's avatar
      AioContext: fix broken placement of event_notifier_test_and_clear · 21a03d17
      Paolo Bonzini authored 
      
event_notifier_test_and_clear must be called before processing events.
Otherwise, an aio_poll could "eat" the notification before the main
I/O thread invokes ppoll().  The main I/O thread then never wakes up.
This is an example of what could happen:

   i/o thread       vcpu thread                     worker thread
   ---------------------------------------------------------------------
   lock_iothread
   notify_me = 1
   ...
   unlock_iothread
                                                     bh->scheduled = 1
                                                     event_notifier_set
                    lock_iothread
                    notify_me = 3
                    ppoll
                    notify_me = 1
                    aio_dispatch
                     aio_bh_poll
                      thread_pool_completion_bh
                                                     bh->scheduled = 1
                                                     event_notifier_set
                     node->io_read(node->opaque)
                      event_notifier_test_and_clear
   ppoll
   *** hang ***

"Tracing" with qemu_clock_get_ns shows pretty much the same behavior as
in the previous bug, so there are no new tricks here---just stare more
at the code until it is apparent.

One could also use a formal model, of course.  The included one shows
this with three processes: notifier corresponds to a QEMU thread pool
worker, temporary_waiter to a VCPU thread that invokes aio_poll(),
waiter to the main I/O thread.  I would be happy to say that the
formal model found the bug for me, but actually I wrote it after the
fact.

This patch is a bit of a big hammer.  The next one optimizes it,
with help (this time for real rather than a posteriori :)) from
another, similar formal model.

Reported-by: default avatarRichard W. M. Jones <rjones@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Tested-by: default avatarRichard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-6-git-send-email-pbonzini@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      21a03d17
    • Paolo Bonzini's avatar
      AioContext: fix broken ctx->dispatching optimization · eabc9779
      Paolo Bonzini authored 
      
This patch rewrites the ctx->dispatching optimization, which was the cause
of some mysterious hangs that could be reproduced on aarch64 KVM only.
The hangs were indirectly caused by aio_poll() and in particular by
flash memory updates's call to blk_write(), which invokes aio_poll().
Fun stuff: they had an extremely short race window, so much that
adding all kind of tracing to either the kernel or QEMU made it
go away (a single printf made it half as reproducible).

On the plus side, the failure mode (a hang until the next keypress)
made it very easy to examine the state of the process with a debugger.
And there was a very nice reproducer from Laszlo, which failed pretty
often (more than half of the time) on any version of QEMU with a non-debug
kernel; it also failed fast, while still in the firmware.  So, it could
have been worse.

For some unknown reason they happened only with virtio-scsi, but
that's not important.  It's more interesting that they disappeared with
io=native, making thread-pool.c a likely suspect for where the bug arose.
thread-pool.c is also one of the few places which use bottom halves
across threads, by the way.

I hope that no other similar bugs exist, but just in case :) I am
going to describe how the successful debugging went...  Since the
likely culprit was the ctx->dispatching optimization, which mostly
affects bottom halves, the first observation was that there are two
qemu_bh_schedule() invocations in the thread pool: the one in the aio
worker and the one in thread_pool_completion_bh.  The latter always
causes the optimization to trigger, the former may or may not.  In
order to restrict the possibilities, I introduced new functions
qemu_bh_schedule_slow() and qemu_bh_schedule_fast():

     /* qemu_bh_schedule_slow: */
     ctx = bh->ctx;
     bh->idle = 0;
     if (atomic_xchg(&bh->scheduled, 1) == 0) {
         event_notifier_set(&ctx->notifier);
     }

     /* qemu_bh_schedule_fast: */
     ctx = bh->ctx;
     bh->idle = 0;
     assert(ctx->dispatching);
     atomic_xchg(&bh->scheduled, 1);

Notice how the atomic_xchg is still in qemu_bh_schedule_slow().  This
was already debated a few months ago, so I assumed it to be correct.
In retrospect this was a very good idea, as you'll see later.

Changing thread_pool_completion_bh() to qemu_bh_schedule_fast() didn't
trigger the assertion (as expected).  Changing the worker's invocation
to qemu_bh_schedule_slow() didn't hide the bug (another assumption
which luckily held).  This already limited heavily the amount of
interaction between the threads, hinting that the problematic events
must have triggered around thread_pool_completion_bh().

As mentioned early, invoking a debugger to examine the state of a
hung process was pretty easy; the iothread was always waiting on a
poll(..., -1) system call.  Infinite timeouts are much rarer on x86,
and this could be the reason why the bug was never observed there.
With the buggy sequence more or less resolved to an interaction between
thread_pool_completion_bh() and poll(..., -1), my "tracing" strategy was
to just add a few qemu_clock_get_ns(QEMU_CLOCK_REALTIME) calls, hoping
that the ordering of aio_ctx_prepare(), aio_ctx_dispatch, poll() and
qemu_bh_schedule_fast() would provide some hint.  The output was:

    (gdb) p last_prepare
    $3 = 103885451
    (gdb) p last_dispatch
    $4 = 103876492
    (gdb) p last_poll
    $5 = 115909333
    (gdb) p last_schedule
    $6 = 115925212

Notice how the last call to qemu_poll_ns() came after aio_ctx_dispatch().
This makes little sense unless there is an aio_poll() call involved,
and indeed with a slightly different instrumentation you can see that
there is one:

    (gdb) p last_prepare
    $3 = 107569679
    (gdb) p last_dispatch
    $4 = 107561600
    (gdb) p last_aio_poll
    $5 = 110671400
    (gdb) p last_schedule
    $6 = 110698917

So the scenario becomes clearer:

   iothread                   VCPU thread
--------------------------------------------------------------------------
   aio_ctx_prepare
   aio_ctx_check
   qemu_poll_ns(timeout=-1)
                              aio_poll
                                aio_dispatch
                                  thread_pool_completion_bh
                                    qemu_bh_schedule()

At this point bh->scheduled = 1 and the iothread has not been woken up.
The solution must be close, but this alone should not be a problem,
because the bottom half is only rescheduled to account for rare situations
(see commit 3c80ca15, thread-pool: avoid deadlock in nested aio_poll()
calls, 2014-07-15).

Introducing a third thread---a thread pool worker thread, which
also does qemu_bh_schedule()---does bring out the problematic case.
The third thread must be awakened *after* the callback is complete and
thread_pool_completion_bh has redone the whole loop, explaining the
short race window.  And then this is what happens:

                                                      thread pool worker
--------------------------------------------------------------------------
                                                      <I/O completes>
                                                      qemu_bh_schedule()

Tada, bh->scheduled is already 1, so qemu_bh_schedule() does nothing
and the iothread is never woken up.  This is where the bh->scheduled
optimization comes into play---it is correct, but removing it would
have masked the bug.

So, what is the bug?

Well, the question asked by the ctx->dispatching optimization ("is any
active aio_poll dispatching?") was wrong.  The right question to ask
instead is "is any active aio_poll *not* dispatching", i.e. in the prepare
or poll phases?  In that case, the aio_poll is sleeping or might go to
sleep anytime soon, and the EventNotifier must be invoked to wake
it up.

In any other case (including if there is *no* active aio_poll at all!)
we can just wait for the next prepare phase to pick up the event (e.g. a
bottom half); the prepare phase will avoid the blocking and service the
bottom half.

Expressing the invariant with a logic formula, the broken one looked like:

   !(exists(thread): in_dispatching(thread)) => !optimize

or equivalently:

   !(exists(thread):
          in_aio_poll(thread) && in_dispatching(thread)) => !optimize

In the correct one, the negation is in a slightly different place:

   (exists(thread):
         in_aio_poll(thread) && !in_dispatching(thread)) => !optimize

or equivalently:

   (exists(thread): in_prepare_or_poll(thread)) => !optimize

Even if the difference boils down to moving an exclamation mark :)
the implementation is quite different.  However, I think the new
one is simpler to understand.

In the old implementation, the "exists" was implemented with a boolean
value.  This didn't really support well the case of multiple concurrent
event loops, but I thought that this was okay: aio_poll holds the
AioContext lock so there cannot be concurrent aio_poll invocations, and
I was just considering nested event loops.  However, aio_poll _could_
indeed be concurrent with the GSource.  This is why I came up with the
wrong invariant.

In the new implementation, "exists" is computed simply by counting how many
threads are in the prepare or poll phases.  There are some interesting
points to consider, but the gist of the idea remains:

1) AioContext can be used through GSource as well; as mentioned in the
patch, bit 0 of the counter is reserved for the GSource.

2) the counter need not be updated for a non-blocking aio_poll, because
it won't sleep forever anyway.  This is just a matter of checking
the "blocking" variable.  This requires some changes to the win32
implementation, but is otherwise not too complicated.

3) as mentioned above, the new implementation will not call aio_notify
when there is *no* active aio_poll at all.  The tests have to be
adjusted for this change.  The calls to aio_notify in async.c are fine;
they only want to kick aio_poll out of a blocking wait, but need not
do anything if aio_poll is not running.

4) nested aio_poll: these just work with the new implementation; when
a nested event loop is invoked, the outer event loop is never in the
prepare or poll phases.  The outer event loop thus has already decremented
the counter.

Reported-by: default avatarRichard W. M. Jones <rjones@redhat.com>
Reported-by: default avatarLaszlo Ersek <lersek@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Tested-by: default avatarRichard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-5-git-send-email-pbonzini@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      eabc9779
    • Paolo Bonzini's avatar
      aio-win32: reorganize polling loop · 6493c975
      Paolo Bonzini authored 
      
Preparatory bugfixes and tweaks to the loop before the next patch:

- disable dispatch optimization during aio_prepare.  This fixes a bug.

- do not modify "blocking" until after the first WaitForMultipleObjects
call.  This is needed in the next patch.

- change the loop to do...while.  This makes it obvious that the loop
is always entered at least once.  In the next patch this is important
because the first iteration undoes the ctx->notify_me increment that
happened before entering the loop.

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Tested-by: default avatarRichard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-4-git-send-email-pbonzini@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      6493c975
  19. Apr 28, 2015
  21. Dec 10, 2014
  23. Sep 22, 2014
  25. Aug 29, 2014
  27. Jul 09, 2014
  29. Dec 06, 2013
    • Stefan Hajnoczi's avatar
      aio: make aio_poll(ctx, true) block with no fds · d3fa9230
      Stefan Hajnoczi authored 
      
This patch drops a special case where aio_poll(ctx, true) returns false
instead of blocking if no file descriptors are waiting on I/O.  Now it
is possible to block in aio_poll() to wait for aio_notify().

This change eliminates busy waiting.  bdrv_drain_all() used to rely on
busy waiting to completed throttled I/O requests but this is no longer
required so we can simplify aio_poll().

Note that aio_poll() still returns false when aio_notify() was used.  In
other words, stopping a blocking aio_poll() wait is not considered
making progress.

Adjust test-aio /aio/bh/callback-delete/one which assumed aio_poll(ctx,
true) would immediately return false instead of blocking.

Reviewed-by: default avatarAlex Bligh <alex@alex.org.uk>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      d3fa9230
  31. Aug 22, 2013
  33. Aug 19, 2013
    • Stefan Hajnoczi's avatar
      aio: drop io_flush argument · f2e5dca4
      Stefan Hajnoczi authored 
      
The .io_flush() handler no longer exists and has no users.  Drop the
io_flush argument to aio_set_fd_handler() and related functions.

The AioFlushEventNotifierHandler and AioFlushHandler typedefs are no
longer used and are dropped too.

Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      f2e5dca4
    • Stefan Hajnoczi's avatar
      aio: stop using .io_flush() · 164a101f
      Stefan Hajnoczi authored 
      
Now that aio_poll() users check their termination condition themselves,
it is no longer necessary to call .io_flush() handlers.

The behavior of aio_poll() changes as follows:

1. .io_flush() is no longer invoked and file descriptors are *always*
monitored.  Previously returning 0 from .io_flush() would skip this file
descriptor.

Due to this change it is essential to check that requests are pending
before calling qemu_aio_wait().  Failure to do so means we block, for
example, waiting for an idle iSCSI socket to become readable when there
are no requests.  Currently all qemu_aio_wait()/aio_poll() callers check
before calling.

2. aio_poll() now returns true if progress was made (BH or fd handlers
executed) and false otherwise.  Previously it would return true whenever
'busy', which means that .io_flush() returned true.  The 'busy' concept
no longer exists so just progress is returned.

Due to this change we need to update tests/test-aio.c which asserts
aio_poll() return values.  Note that QEMU doesn't actually rely on these
return values so only tests/test-aio.c cares.

Note that ctx->notifier, the EventNotifier fd used for aio_notify(), is
now handled as a special case.  This is a little ugly but maintains
aio_poll() semantics, i.e. aio_notify() does not count as 'progress' and
aio_poll() avoids blocking when the user has not set any fd handlers yet.

Patches after this remove .io_flush() handler code until we can finally
drop the io_flush arguments to aio_set_fd_handler() and friends.

Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
      164a101f