Re: [Qemu-devel] [Qemu-block] [PATCH] blockjob: kick jobs on set-speed

[Stefan Hajnoczi]

On Tue, Dec 12, 2017 at 01:22:28PM -0500, John Snow wrote:
> 
> 
> On 12/11/2017 07:08 PM, Jeff Cody wrote:
> > On Mon, Dec 11, 2017 at 06:46:09PM -0500, John Snow wrote:
> >> If users set an unreasonably low speed (like one byte per second), the
> >> calculated delay may exceed many hours. While we like to punish users
> >> for asking for stupid things, we do also like to allow users to correct
> >> their wicked ways.
> >>
> >> When a user provides a new speed, kick the job to allow it to recalculate
> >> its delay.
> >>
> >> Signed-off-by: John Snow <address@hidden>
> >> ---
> >>  blockjob.c | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >> diff --git a/blockjob.c b/blockjob.c
> >> index 715c2c2680..43f01ad190 100644
> >> --- a/blockjob.c
> >> +++ b/blockjob.c
> >> @@ -483,6 +483,7 @@ static void block_job_completed_txn_success(BlockJob 
> >> *job)
> >>  void block_job_set_speed(BlockJob *job, int64_t speed, Error **errp)
> >>  {
> >>      Error *local_err = NULL;
> >> +    int64_t old_speed = job->speed;
> >>  
> >>      if (!job->driver->set_speed) {
> >>          error_setg(errp, QERR_UNSUPPORTED);
> >> @@ -495,6 +496,10 @@ void block_job_set_speed(BlockJob *job, int64_t 
> >> speed, Error **errp)
> >>      }
> >>  
> >>      job->speed = speed;
> >> +    /* Kick the job to recompute its delay */
> >> +    if ((speed > old_speed) && timer_pending(&job->sleep_timer)) {
> > 
> > job->sleep_timer is protected by block_job_mutex (via
> > block_job_lock/unlock); is it safe for us to check it here outside the
> > mutex?
> > 
> 
> My hunch is that in this specific case that it is; but only because of
> assumptions about holding the aio_context and the QEMU global mutex here.
> 
> > But in any case, I think we could get rid of the timer_pending check, and
> > just always kick the job if we have a speed increase.  block_job_enter()
> > should do the right thing (mutex protected check on job->busy and
> > job->sleep_timer).
> > 
> 
> I could lock it for inarguable correctness; I just didn't want to kick a
> job that didn't actually require any kicking to limit any potential
> problems from that interaction.
> 
> (I'm fond of the extra conditional because I feel like it makes the
> intent of the kick explicit.)
> 
> I can remove it.

Removing the conditional would introduce a bug.  block_job_enter() will
unpause the job.

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