Re: [Qemu-devel] [PATCH] block/throttle: Use host clock type

[Fam Zheng]

On Fri, 03/13 09:08, Paolo Bonzini wrote:
> 
> 
> On 13/03/2015 07:35, Fam Zheng wrote:
> > Throttle timers won't make any progress when VCPU is not running, which
> > is prone to stall the request queue in cases like utils, qtest,
> > suspending, and live migration, unless carefully handled. What we do now
> > is crude. For example in bdrv_drain_all, requests are resumed
> > immediately without consulting throttling timer. Unfortunately
> > bdrv_drain_all is so widely used that there may be too many holes that
> > guest could bypass throttling.
> > 
> > If we use the host clock, we can just trust the nested poll when waiting
> > for requests.
> > 
> > Signed-off-by: Fam Zheng <address@hidden>
> > ---
> >  block.c               |  2 +-
> >  tests/test-throttle.c | 14 +++++++-------
> 
> I think test-throttle.c should use the vm_clock.  At some point it was
> managing the clock manually (by overriding cpu_get_clock from
> libqemustub.a), and that's only possible with QEMU_CLOCK_VIRTUAL.

Ah! That is in iotests 093 (hint: authord by Fam Zheng :-/), which WILL be
complicated if block.c switches away from QEMU_CLOCK_VIRTUAL. But I'll do the
work if we decide to make this change.

As to tests/test-throttle.c, I don't see its dependency on clock type, so
either way should work and I don't mind keeping it as-is at all.

> 
> As to block.c, I'll leave the review to the block folks.  But I think
> QEMU_CLOCK_REALTIME is preferrable.

Real time clock should be fine, but we should review that the code handles
clock reversing.

Fam

> 
> >  2 files changed, 8 insertions(+), 8 deletions(-)
> > 
> > diff --git a/block.c b/block.c
> > index 191a847..11f9065 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -184,7 +184,7 @@ void bdrv_io_limits_enable(BlockDriverState *bs)
> >      assert(!bs->io_limits_enabled);
> >      throttle_init(&bs->throttle_state,
> >                    bdrv_get_aio_context(bs),
> > -                  QEMU_CLOCK_VIRTUAL,
> > +                  QEMU_CLOCK_HOST,
> >                    bdrv_throttle_read_timer_cb,
> >                    bdrv_throttle_write_timer_cb,
> >                    bs);
> > diff --git a/tests/test-throttle.c b/tests/test-throttle.c
> > index d8ba415..1fb1792 100644
> > --- a/tests/test-throttle.c
> > +++ b/tests/test-throttle.c
> > @@ -107,11 +107,11 @@ static void test_init(void)
> >      memset(&ts, 1, sizeof(ts));
> >  
> >      /* init the structure */
> > -    throttle_init(&ts, ctx, QEMU_CLOCK_VIRTUAL,
> > +    throttle_init(&ts, ctx, QEMU_CLOCK_HOST,
> >                    read_timer_cb, write_timer_cb, &ts);
> >  
> >      /* check initialized fields */
> > -    g_assert(ts.clock_type == QEMU_CLOCK_VIRTUAL);
> > +    g_assert(ts.clock_type == QEMU_CLOCK_HOST);
> >      g_assert(ts.timers[0]);
> >      g_assert(ts.timers[1]);
> >  
> > @@ -130,7 +130,7 @@ static void test_init(void)
> >  static void test_destroy(void)
> >  {
> >      int i;
> > -    throttle_init(&ts, ctx, QEMU_CLOCK_VIRTUAL,
> > +    throttle_init(&ts, ctx, QEMU_CLOCK_HOST,
> >                    read_timer_cb, write_timer_cb, &ts);
> >      throttle_destroy(&ts);
> >      for (i = 0; i < 2; i++) {
> > @@ -170,7 +170,7 @@ static void test_config_functions(void)
> >  
> >      orig_cfg.op_size = 1;
> >  
> > -    throttle_init(&ts, ctx, QEMU_CLOCK_VIRTUAL,
> > +    throttle_init(&ts, ctx, QEMU_CLOCK_HOST,
> >                    read_timer_cb, write_timer_cb, &ts);
> >      /* structure reset by throttle_init previous_leak should be null */
> >      g_assert(!ts.previous_leak);
> > @@ -330,7 +330,7 @@ static void test_have_timer(void)
> >      g_assert(!throttle_have_timer(&ts));
> >  
> >      /* init the structure */
> > -    throttle_init(&ts, ctx, QEMU_CLOCK_VIRTUAL,
> > +    throttle_init(&ts, ctx, QEMU_CLOCK_HOST,
> >                    read_timer_cb, write_timer_cb, &ts);
> >  
> >      /* timer set by init should return true */
> > @@ -345,7 +345,7 @@ static void test_detach_attach(void)
> >      memset(&ts, 0, sizeof(ts));
> >  
> >      /* init the structure */
> > -    throttle_init(&ts, ctx, QEMU_CLOCK_VIRTUAL,
> > +    throttle_init(&ts, ctx, QEMU_CLOCK_HOST,
> >                    read_timer_cb, write_timer_cb, &ts);
> >  
> >      /* timer set by init should return true */
> > @@ -387,7 +387,7 @@ static bool do_test_accounting(bool is_ops, /* are we 
> > testing bps or ops */
> >  
> >      cfg.op_size = op_size;
> >  
> > -    throttle_init(&ts, ctx, QEMU_CLOCK_VIRTUAL,
> > +    throttle_init(&ts, ctx, QEMU_CLOCK_HOST,
> >                    read_timer_cb, write_timer_cb, &ts);
> >      throttle_config(&ts, &cfg);
> >  
> >