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SubjectRe: [PATCH RFT] blk-mq: optimize queue tag busy iter for shared_tags
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On 21/12/2021 12:31, Kashyap Desai wrote:

Hi Kashyap,

What kernel is this for? 5.17 or 5.16 + stable? Your intention is not
clear to me.


> In [0], CPU usage for blk_mq_queue_tag_busy_iter() was optimized, but
> there are still periodic call of blk_mq_queue_tag_busy_iter() from
> below context. Below context is used for block layer timer to find out
> potential expired command (per request queue) which requires tag iteration
> almost every 5 seconds(defined BLK_MAX_TIMEOUT) for each request queue.
>
> kthread
> worker_thread
> process_one_work
> blk_mq_timeout_work
> blk_mq_queue_tag_busy_iter
> bt_iter
> blk_mq_find_and_get_req
> _raw_spin_lock_irqsave
> native_queued_spin_lock_slowpath
>
> Changes in this patch optimize extra iterations of tags in case of
> shared_tags. One iteration of shared_tags can give expected results for
> iterate function.
>
> Setup - AMD64 Gen-4.0 Server.
> 64 Virtual Drive created using 16 Nvme drives + mpi3mr driver (in
> shared_tags mode)
>
> Test command -
> fio 64.fio --rw=randread --bs=4K --iodepth=32 --numjobs=2 --ioscheduler=mq-deadline --disk_util=0
>
> Without this patch on 5.16.0-rc5, mpi3mr driver in shared_tags mode can
> give 4.0M IOPs vs expected to get ~6.0M.
> Snippet of perf top
>
> 25.42% [kernel] [k] native_queued_spin_lock_slowpath
> 3.95% [kernel] [k] cpupri_set
> 2.05% [kernel] [k] __blk_mq_get_driver_tag
> 1.67% [kernel] [k] __rcu_read_unlock
> 1.63% [kernel] [k] check_preemption_disabled
>
> After applying this patch on 5.16.0-rc5, mpi3mr driver in shared_tags
> mode reach up to 5.8M IOPs.
>
> Snippet of perf top
>
> 7.95% [kernel] [k] native_queued_spin_lock_slowpath
> 5.61% [kernel] [k] cpupri_set
> 2.98% [kernel] [k] acpi_processor_ffh_cstate_enter
> 2.49% [kernel] [k] read_tsc
> 2.15% [kernel] [k] check_preemption_disabled
>
>
> [0] https://lore.kernel.org/all/9b092ca49e9b5415772cd950a3c12584@mail.gmail.com/
>
>
> Cc: linux-block@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: john.garry@huawei.com
> Cc: ming.lei@redhat.com
> Cc: sathya.prakash@broadcom.com
> Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
> ---
> block/blk-mq-tag.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 995336abee33..3e0a8e79f966 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -253,7 +253,8 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> if (!rq)
> return true;
>
> - if (rq->q == hctx->queue && rq->mq_hctx == hctx)
> + if (rq->q == hctx->queue && (rq->mq_hctx == hctx ||
> + blk_mq_is_shared_tags(hctx->flags)))
> ret = iter_data->fn(hctx, rq, iter_data->data, reserved);
> blk_mq_put_rq_ref(rq);
> return ret;
> @@ -484,6 +485,14 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
> if (tags->nr_reserved_tags)
> bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
> bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
> +
> + /* In case of shared bitmap if shared_tags is allocated, it is not required
> + * to iterate all the hctx. Looping one hctx is good enough.
> + */
> + if (blk_mq_is_shared_tags(hctx->flags)) {
> + blk_queue_exit(q);
> + return;

this looks like v5.16-rc6 code

> + }
> }
> blk_queue_exit(q);
> }
>



Thanks,
John

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