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

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Message-ID: <97f406a5-ca2c-43a4-d7a1-e6cf40b7a917@kernel.dk>
Date:   Tue, 16 Jan 2018 08:56:25 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Tejun Heo <tj@...nel.org>, Shaohua Li <shli@...com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RESEND] blkcg: simplify statistic accumulation code

On 1/16/18 8:01 AM, 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.

Added for 4.16, thanks Arnd.

-- 
Jens Axboe