linux-kernel - Re: [PATCH] blkcg: simplify statistic accumulation code

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Message-ID: <20170919032655.GO378890@devbig577.frc2.facebook.com>
Date:   Mon, 18 Sep 2017 20:26:55 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jens Axboe <axboe@...com>, Tahsin Erdogan <tahsin@...gle.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Shaohua Li <shli@...com>, Nikolay Borisov <nborisov@...e.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blkcg: simplify statistic accumulation code

On Fri, Sep 15, 2017 at 09:36:21PM +0200, Arnd Bergmann wrote:
> Some older compilers (gcc-4.4 through 4.6 in particular) struggle
> with the way that blkg_rwstat_read() returns a structure, leading
> to excessive stack usage and rather inefficient code:
> 
> block/blk-cgroup.c: In function 'blkg_destroy':
> block/blk-cgroup.c:354:1: error: the frame size of 1296 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
> block/cfq-iosched.c: In function 'cfqg_stats_add_aux':
> block/cfq-iosched.c:753:1: error: the frame size of 1928 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
> block/bfq-cgroup.c: In function 'bfqg_stats_add_aux':
> block/bfq-cgroup.c:299:1: error: the frame size of 1928 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]
> 
> I also notice that there is no point in using atomic accesses
> for the local variables, so storing the temporaries in simple 'u64'
> variables not only avoids the stack usage on older compilers but
> also improves the object code on modern versions.
> 
> Fixes: e6269c445467 ("blkcg: add blkg_[rw]stat->aux_cnt and replace cfq_group->dead_stats with it")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun