linux-kernel - RE: [PATCH 8/8] mm/zswap: Use local lock to protect per-CPU data

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Message-ID: <B926444035E5E2439431908E3842AFD24B2155@DGGEMI525-MBS.china.huawei.com>
Date:   Wed, 20 May 2020 12:01:37 +0000
From:   Song Bao Hua <song.bao.hua@...ilicon.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linuxarm <linuxarm@...wei.com>
Subject: RE: [PATCH 8/8] mm/zswap: Use local lock to protect per-CPU data

> Subject: Re: [PATCH 8/8] mm/zswap: Use local lock to protect per-CPU data
> 
> On 2020-05-20 11:13:31 [+0000], Song Bao Hua wrote:
> > For example, on cpu1, once you begin to compress, you hold the percpu
> acomp-ctx and percpu destination buffer of CPU1, the below code makes sure
> you get the acomp and dstmem from the same core by disabling preemption
> with get_cpu_var and put_cpu_var:
> > dst = get_cpu_var(zswap_dstmem);
> > acomp_ctx = *this_cpu_ptr(entry->pool->acomp_ctx);
> > put_cpu_var(zswap_dstmem);
> >
> > then there are two cases:
> >
> > 1. after getting dst and acomp_ctx of cpu1, you might always work in cpu1,
> the mutex in per-cpu acomp-ctx will guarantee two compressions won't do at
> the same core in parallel, and it also makes certain compression and
> decompression won't do at the same core in parallel. Everything is like before.
> 
> For readability I suggest not to mix per-CPU and per-CTX variables like that. If
> zswap_dstmem is protected by the mutex, please make it part of acomp_ctx.
> 

Yep. it seems this will avoid the further explanations to more people who will read the code :-)

> Sebastian

Barry