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

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Date:   Fri, 4 Oct 2019 15:24:43 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     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 15.10, 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?
> 

No, this is completely different semantics.

Operation could be safely skipped only if somebody else started and
finished drain after current task called this function.