Commit Graph

6 Commits

Author SHA1 Message Date
Paolo Bonzini 6b8f0187a4 icount: process QEMU_CLOCK_VIRTUAL timers in vCPU thread
icount has become much slower after tcg_cpu_exec has stopped
using the BQL.  There is also a latent bug that is masked by
the slowness.

The slowness happens because every occurrence of a QEMU_CLOCK_VIRTUAL
timer now has to wake up the I/O thread and wait for it.  The rendez-vous
is mediated by the BQL QemuMutex:

- handle_icount_deadline wakes up the I/O thread with BQL taken
- the I/O thread wakes up and waits on the BQL
- the VCPU thread releases the BQL a little later
- the I/O thread raises an interrupt, which calls qemu_cpu_kick
- the VCPU thread notices the interrupt, takes the BQL to
  process it and waits on it

All this back and forth is extremely expensive, causing a 6 to 8-fold
slowdown when icount is turned on.

One may think that the issue is that the VCPU thread is too dependent
on the BQL, but then the latent bug comes in.  I first tried removing
the BQL completely from the x86 cpu_exec, only to see everything break.
The only way to fix it (and make everything slow again) was to add a dummy
BQL lock/unlock pair.

This is because in -icount mode you really have to process the events
before the CPU restarts executing the next instruction.  Therefore, this
series moves the processing of QEMU_CLOCK_VIRTUAL timers straight in
the vCPU thread when running in icount mode.

The required changes include:

- make the timer notification callback wake up TCG's single vCPU thread
  when run from another thread.  By using async_run_on_cpu, the callback
  can override all_cpu_threads_idle() when the CPU is halted.

- move handle_icount_deadline after qemu_tcg_wait_io_event, so that
  the timer notification callback is invoked after the dummy work item
  wakes up the vCPU thread

- make handle_icount_deadline run the timers instead of just waking the
  I/O thread.

- stop processing the timers in the main loop

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2017-03-14 13:51:34 +01:00
Paolo Bonzini 3f53bc61a4 cpus: define QEMUTimerListNotifyCB for QEMU system emulation
There is no change for now, because the callback just invokes
qemu_notify_event.

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2017-03-14 13:28:29 +01:00
Paolo Bonzini d2528bdc19 qemu-timer: do not include sysemu/cpus.h from util/qemu-timer.h
This dependency is the wrong way, and we will need util/qemu-timer.h from
sysemu/cpus.h in the next patch.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2017-03-14 13:28:18 +01:00
Paolo Bonzini 33bef0b994 qemu-timer: fix off-by-one
If the first timer is exactly at the current value of the clock, the
deadline is met and the timer should fire.  This fixes itself on the next
iteration of the loop without icount; with icount, however, execution
of instructions will stop exactly at the deadline and won't proceed.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2017-03-14 13:26:42 +01:00
Marc-André Lureau e703dcbaee timer: use an inline function for free
Similarly to allocation, do it from an inline function. This allows
tests to only use the headers for allocation/free of timer.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
2017-03-01 00:09:28 +04:00
Paolo Bonzini c2b38b277a block: move AioContext, QEMUTimer, main-loop to libqemuutil
AioContext is fairly self contained, the only dependency is QEMUTimer but
that in turn doesn't need anything else.  So move them out of block-obj-y
to avoid introducing a dependency from io/ to block-obj-y.

main-loop and its dependency iohandler also need to be moved, because
later in this series io/ will call iohandler_get_aio_context.

[Changed copyright "the QEMU team" to "other QEMU contributors" as
suggested by Daniel Berrange and agreed by Paolo.
--Stefan]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170213135235.12274-2-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-02-21 11:14:07 +00:00