On Tue, Dec 14, 2021 at 07:53:46AM -0700, Jens Axboe wrote: > Dexuan reports that he's seeing spikes of very heavy CPU utilization when > running 24 disks and using the 'none' scheduler. This happens off the > flush path, because SCSI requires the queue to be restarted async, and > hence we're hammering on mod_delayed_work_on() to ensure that the work > item gets run appropriately. > > What we care about here is that the queue is run, and we don't need to > repeatedly re-arm the timer associated with the delayed work item. If we > check if the work item is pending upfront, then we don't really need to do > anything else. This is safe as theh work pending bit is cleared before a > work item is started. > > The only potential caveat here is if we have callers with wildly different > timeouts specified. That's generally not the case, so don't think we need > to care for that case. So why not do a non-delayed queue_work for that case? Might be good to get the scsi and workqueue maintaines involved to understand the issue a bit better first. > > Reported-by: Dexuan Cui <decui@xxxxxxxxxxxxx> > Link: https://lore.kernel.org/linux-block/BYAPR21MB1270C598ED214C0490F47400BF719@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> > > --- > > diff --git a/block/blk-core.c b/block/blk-core.c > index 1378d084c770..4584fe709c15 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -1484,7 +1484,16 @@ EXPORT_SYMBOL(kblockd_schedule_work); > int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork, > unsigned long delay) > { > - return mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay); > + /* > + * Avoid hammering on work addition, if the work item is already > + * pending. This is safe the work pending state is cleared before > + * the work item is started, so if we see it set, then we know that > + * whatever was previously queued on the block side will get run by > + * an existing pending work item. > + */ > + if (!work_pending(&dwork->work)) > + return mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay); > + return true; > } > EXPORT_SYMBOL(kblockd_mod_delayed_work_on); > > -- > Jens Axboe > ---end quoted text---
