Re: [PATCH -V3] swap: Reduce lock contention on swap cache from swap slots allocation

[Huang\, Ying]

Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:

> On Thu, May 28, 2020 at 01:32:40PM +0800, Huang, Ying wrote:
>> Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:
>> 
>> > On Mon, May 25, 2020 at 08:26:48AM +0800, Huang Ying wrote:
>> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> >> index 423c234aca15..0abd93d2a4fc 100644
>> >> --- a/mm/swapfile.c
>> >> +++ b/mm/swapfile.c
>> >> @@ -615,7 +615,8 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>> >>  			 * discarding, do discard now and reclaim them
>> >>  			 */
>> >>  			swap_do_scheduled_discard(si);
>> >> -			*scan_base = *offset = si->cluster_next;
>> >> +			*scan_base = this_cpu_read(*si->cluster_next_cpu);
>> >> +			*offset = *scan_base;
>> >>  			goto new_cluster;
>> >
>> > Why is this done?  As far as I can tell, the values always get overwritten at
>> > the end of the function with tmp and tmp isn't derived from them.  Seems
>> > ebc2a1a69111 moved some logic that used to make sense but doesn't have any
>> > effect now.
>> 
>> If we fail to allocate from cluster, "scan_base" and "offset" will not
>> be overridden.
>
> Ok, if another task races to allocate the clusters the first just discarded.
>
>> And "cluster_next" or "cluster_next_cpu" may be changed
>> in swap_do_scheduled_discard(), because the lock is released and
>> re-acquired there.
>
> I see, by another task on the same cpu for cluster_next_cpu.
>
> Both probably unlikely, but at least it tries to pick up where the racing task
> left off.  You might tack this onto the comment:
>
> 		 * discarding, do discard now and reclaim them, then reread
>                  * cluster_next_cpu since we dropped si->lock
>                 /*

Sure.  Will add this in the next version.

>> The code may not have much value.
>
> No, it makes sense.
>
>> > These aside, patch looks good to me.
>> 
>> Thanks for your review!  It really help me to improve the quality of the
>> patch.  Can I add your "Reviewed-by" in the next version?
>
> Sure,
> Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>

Thanks!

Best Regards,
Huang, Ying