Re: [PATCH] bcache: add cond_resched() in __bch_cache_cmp()

[Shile Zhang]

On 2019/3/7 18:34, Coly Li wrote:
> On 2019/3/7 1:15 äå, shile.zhang@xxxxxxxxxxxxxxxxx wrote:
> > From: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
> >
> > Read /sys/fs/bcache/<uuid>/cacheN/priority_stats can take very long
> > time with huge cache after long run.
> >
> > Signed-off-by: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
> Hi Shile,
>
> Do you test your change ? It will be helpful with more performance data
> (what problem that you improved).

In case of 960GB SSD cache device, once read of the 'priority_stats' 
costs about 600ms in our test environment.

The perf tool shown that near 50% CPU time consumed by 'sort()', this 
means once sort will hold the CPU near 300ms.

In our case, the statistics collector reads the 'priority_stats' 
periodically, it will trigger the schedule latency jitters of the

task which shared same CPU core.

> Thanks.
>
> Coly Li
>
> > ---
> >   drivers/md/bcache/sysfs.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> > index 557a8a3..028fea1 100644
> > --- a/drivers/md/bcache/sysfs.c
> > +++ b/drivers/md/bcache/sysfs.c
> > @@ -897,6 +897,7 @@ static void bch_cache_set_internal_release(struct kobject *k)
> >   
> >   static int __bch_cache_cmp(const void *l, const void *r)
> >   {
> > +	cond_resched();
> >   	return *((uint16_t *)r) - *((uint16_t *)l);
> >   }
> >