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

[John Snow]

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.

>> +        block_job_enter(job);
>> +    }
>>  }
>>  
>>  void block_job_complete(BlockJob *job, Error **errp)
>> -- 
>> 2.14.3
>>
>