linux-kernel - Re: [PATCH] mm/swap: piggyback lru_add_drain

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Date:   Fri, 4 Oct 2019 15:32:01 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Michal Hocko <mhocko@...nel.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls



On 04/10/2019 15.27, Michal Hocko wrote:
> On Fri 04-10-19 05:10:17, Matthew Wilcox wrote:
>> On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
>>> This is very slow operation. There is no reason to do it again if somebody
>>> else already drained all per-cpu vectors after we waited for lock.
>>> +	seq = raw_read_seqcount_latch(&seqcount);
>>> +
>>>   	mutex_lock(&lock);
>>> +
>>> +	/* Piggyback on drain done by somebody else. */
>>> +	if (__read_seqcount_retry(&seqcount, seq))
>>> +		goto done;
>>> +
>>> +	raw_write_seqcount_latch(&seqcount);
>>> +
>>
>> Do we really need the seqcount to do this?  Wouldn't a mutex_trylock()
>> have the same effect?
> 
> Yeah, this makes sense. From correctness point of view it should be ok
> because no caller can expect that per-cpu pvecs are empty on return.
> This might have some runtime effects that some paths might retry more -
> e.g. offlining path drains pcp pvces before migrating the range away, if
> there are pages still waiting for a worker to drain them then the
> migration would fail and we would retry. But this not a correctness
> issue.
> 

Caller might expect that pages added by him before are drained.
Exiting after mutex_trylock() will not guarantee that.

For example POSIX_FADV_DONTNEED uses that.