Re: [PATCH] blkcg: simplify statistic accumulation code

[Tejun Heo]

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@xxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun