From: "Huang\, Ying" <ying.huang@intel.com>
To: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
<linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>,
Minchan Kim <minchan@kernel.org>,
Tim Chen <tim.c.chen@linux.intel.com>,
Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH -V3] swap: Reduce lock contention on swap cache from swap slots allocation
Date: Fri, 29 May 2020 08:43:35 +0800 [thread overview]
Message-ID: <87d06nh8p4.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <20200528171141.k3lc3mf7taqadv3v@ca-dmjordan1.us.oracle.com> (Daniel Jordan's message of "Thu, 28 May 2020 13:11:41 -0400")
Daniel Jordan <daniel.m.jordan@oracle.com> writes:
> On Thu, May 28, 2020 at 01:32:40PM +0800, Huang, Ying wrote:
>> Daniel Jordan <daniel.m.jordan@oracle.com> 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@oracle.com>
Thanks!
Best Regards,
Huang, Ying
prev parent reply other threads:[~2020-05-29 0:43 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-25 0:26 [PATCH -V3] swap: Reduce lock contention on swap cache from swap slots allocation Huang Ying
2020-05-28 1:37 ` Daniel Jordan
2020-05-28 5:32 ` Huang, Ying
2020-05-28 17:11 ` Daniel Jordan
2020-05-29 0:43 ` Huang, Ying [this message]
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