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Fri, 23 Oct 2020 09:11:24 +0000 (UTC) Received: from T590 (ovpn-12-163.pek2.redhat.com [10.72.12.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 082415B4B3; Fri, 23 Oct 2020 09:11:06 +0000 (UTC) Date: Fri, 23 Oct 2020 17:11:03 +0800 From: Ming Lei To: Weiping Zhang Cc: Jens Axboe , Mike Snitzer , mpatocka@redhat.com, linux-block@vger.kernel.org Subject: Re: [PATCH RFC] block: fix inaccurate io_ticks Message-ID: <20201023091103.GE1698172@T590> References: <20201023064621.GA16839@192.168.3.9> <20201023084653.GD1698172@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Oct 23, 2020 at 04:56:08PM +0800, Weiping Zhang wrote: > On Fri, Oct 23, 2020 at 4:49 PM Ming Lei wrote: > > > > On Fri, Oct 23, 2020 at 02:46:32PM +0800, Weiping Zhang wrote: > > > Do not add io_ticks if there is no infligh io when start a new IO, > > > otherwise an extra 1 jiffy will be add to this IO. > > > > > > I run the following command on a host, with different kernel version. > > > > > > fio -name=test -ioengine=sync -bs=4K -rw=write > > > -filename=/home/test.fio.log -size=100M -time_based=1 -direct=1 > > > -runtime=300 -rate=2m,2m > > > > > > If we run fio in a sync direct io mode, IO will be proccessed one by one, > > > you can see that there are 512 IOs completed in one second. > > > > > > kernel: 4.19.0 > > > > > > Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await r_await w_await svctm %util > > > vda 0.00 0.00 0.00 512.00 0.00 2.00 8.00 0.21 0.40 0.00 0.40 0.40 20.60 > > > > > > The averate io.latency is 0.4ms, so the disk time cost in one second > > > should be 0.4 * 512 = 204.8 ms, that means, %util should be 20%. > > > > > > Becase update_io_ticks will add a extra 1 jiffy(1ms) for every IO, the > > > io.latency io.latency will be 1 + 0.4 = 1.4ms, > > > 1.4 * 512 = 716.8ms, so the %util show it about 72%. > > > > > > Device r/s w/s rMB/s wMB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > > > vda 0.00 512.00 0.00 2.00 0.00 0.00 0.00 0.00 0.00 0.40 0.20 0.00 4.00 1.41 72.10 > > > > > > After this patch: > > > Device r/s w/s rMB/s wMB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util > > > vda 0.00 512.00 0.00 2.00 0.00 0.00 0.00 0.00 0.00 0.40 0.20 0.00 4.00 0.39 20.00 > > > > > > Fixes: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting") > > > Signed-off-by: Weiping Zhang > > > --- > > > block/blk-core.c | 19 ++++++++++++++----- > > > block/blk.h | 1 + > > > block/genhd.c | 2 +- > > > 3 files changed, 16 insertions(+), 6 deletions(-) > > > > > > diff --git a/block/blk-core.c b/block/blk-core.c > > > index ac00d2fa4eb4..789a5c40b6a6 100644 > > > --- a/block/blk-core.c > > > +++ b/block/blk-core.c > > > @@ -1256,14 +1256,14 @@ unsigned int blk_rq_err_bytes(const struct request *rq) > > > } > > > EXPORT_SYMBOL_GPL(blk_rq_err_bytes); > > > > > > -static void update_io_ticks(struct hd_struct *part, unsigned long now, bool end) > > > +static void update_io_ticks(struct hd_struct *part, unsigned long now, bool inflight) > > > { > > > unsigned long stamp; > > > again: > > > stamp = READ_ONCE(part->stamp); > > > if (unlikely(stamp != now)) { > > > - if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) > > > - __part_stat_add(part, io_ticks, end ? now - stamp : 1); > > > + if (likely(cmpxchg(&part->stamp, stamp, now) == stamp) && inflight) > > > + __part_stat_add(part, io_ticks, now - stamp); > > > } > > > if (part->partno) { > > > part = &part_to_disk(part)->part0; > > > @@ -1310,13 +1310,20 @@ void blk_account_io_done(struct request *req, u64 now) > > > > > > void blk_account_io_start(struct request *rq) > > > { > > > + struct hd_struct *part; > > > + struct request_queue *q; > > > + int inflight; > > > + > > > if (!blk_do_io_stat(rq)) > > > return; > > > > > > rq->part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq)); > > > > > > part_stat_lock(); > > > - update_io_ticks(rq->part, jiffies, false); > > > + part = rq->part; > > > + q = part_to_disk(part)->queue; > > > + inflight = blk_mq_in_flight(q, part); > > > + update_io_ticks(part, jiffies, inflight > 0 ? true : false); > > > > Yeah, this account issue can be fixed by applying such 'inflight' info. > > However, blk_mq_in_flight() isn't cheap enough, I did get soft lockup > > report because of blk_mq_in_flight() called in I/O path. > > > > BTW, this way is just like reverting 5b18b5a73760 ("block: delete > > part_round_stats and switch to less precise counting"). > > > > > Hello Ming, > > Shall we switch it to atomic mode ? update inflight_count when > start/done for every IO. That is more expensive than blk_mq_in_flight(). > Or any other cheaper way. I guess it is hard to figure out one cheaper way to figure out IO in-flight count especially in case of multiple CPU cores and Millions of IOPS. Thanks, Ming