On Sun, Feb 16, 2014 at 12:25:29PM +0100, Alexander Gordeev wrote:
> When stealing from a remote CPU all available tags are moved
> from the remote CPU's cache to the stealing CPU's one. Since
> the best CPU to steal from is not selected the victim might
> actively performing IO and (as result of losing all local
> tags) a further cycle of cache rebouncing and/or stealing
> could be provoked.
>
> This update is an attempt to soften the described scenario
> and limit the number of tags to be stolen at once to the
> value of percpu_batch_size.
I don't want this patch without a benchmark justifying the change and more
analysis; this could easily lead to more stealing and cacheline bouncing.
>
> Signed-off-by: Alexander Gordeev <agord...@redhat.com>
> Cc: Kent Overstreet <k...@daterainc.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Jens Axboe <ax...@kernel.dk>
> Cc: "Nicholas A. Bellinger" <n...@linux-iscsi.org>
> ---
> lib/percpu_ida.c | 18 ++++++++++--------
> 1 files changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
> index fad029c..b4c4cc7 100644
> --- a/lib/percpu_ida.c
> +++ b/lib/percpu_ida.c
> @@ -84,20 +84,22 @@ static inline void steal_tags(struct percpu_ida *pool,
> pool->cpu_last_stolen = cpu;
> remote = per_cpu_ptr(pool->tag_cpu, cpu);
>
> - cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
> -
> - if (remote == tags)
> + if (remote == tags) {
> + cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
> continue;
> + }
>
> spin_lock(&remote->lock);
>
> if (remote->nr_free) {
> - memcpy(tags->freelist,
> - remote->freelist,
> - sizeof(unsigned) * remote->nr_free);
> + const struct percpu_ida *p = pool;
> +
> + move_tags(tags->freelist, &tags->nr_free,
> + remote->freelist, &remote->nr_free,
> + min(remote->nr_free, p->percpu_batch_size));
>
> - tags->nr_free = remote->nr_free;
> - remote->nr_free = 0;
> + if (!remote->nr_free)
> + cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
> }
>
> spin_unlock(&remote->lock);
> --
> 1.7.7.6
>
> --
> Regards,
> Alexander Gordeev
> agord...@redhat.com
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