linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Wed, 12 Aug 2020 13:38:35 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Michal Hocko <mhocko@...e.com>, Uladzislau Rezki <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Thomas Gleixner <tglx@...utronix.de> writes:
> Thomas Gleixner <tglx@...utronix.de> writes:
>> Michal Hocko <mhocko@...e.com> writes:
>>> zone->lock should be held for a very limited amount of time.
>>
>> Emphasis on should. free_pcppages_bulk() can hold it for quite some time
>> when a large amount of pages are purged. We surely would have converted
>> it to a raw lock long time ago otherwise.
>>
>> For regular enterprise stuff a few hundred microseconds might qualify as
>> a limited amount of time. For advanced RT applications that's way beyond
>> tolerable..
>
> Sebastian just tried with zone lock converted to a raw lock and maximum
> latencies go up by a factor of 7 when putting a bit of stress on the
> memory subsytem. Just a regular kernel compile kicks them up by a factor
> of 5. Way out of tolerance.
>
> We'll have a look whether it's solely free_pcppages_bulk() and if so we
> could get away with dropping the lock in the loop.

So even on !RT and just doing a kernel compile the time spent in
free_pcppages_bulk() is up to 270 usec.

It's not only the loop which processes a large pile of pages, part of it
is caused by lock contention on zone->lock. Dropping the lock after a
processing a couple of pages does not make it much better if enough CPUs
are contending on the lock.

Thanks,

        tglx