From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ot1-f41.google.com (mail-ot1-f41.google.com [209.85.210.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BCE9D2EAFF for ; Thu, 19 Oct 2023 19:51:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=none Received: by mail-ot1-f41.google.com with SMTP id 46e09a7af769-6cd1918afb2so65644a34.0 for ; Thu, 19 Oct 2023 12:51:46 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1697745106; x=1698349906; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=7U6r7VG/hlgzvJUkfJxMrrqAEusd3ue24ZBPwfHMmPs=; b=IRYRkN27OtHc3YH5L6om10IpGZYSsMV5qdhanwTJKTKQGReDowidytPRWYxCpqRii+ Zoj0QiNOXjaW3Fejx4OeS5uDbzunCcW6ku8BDndhGWpdAKO2y7K6ZqYTBAlJ0mx/OEfv SElfuUgHl4WPWCp6bK+xxsiVUWQbl0etXl1cjUn1pZBDX/P6FsS+7zRi1EfYP6NRcLuC kMhAsm9X8umyHysNxk6DKczxDiSuNsrLe53Ex9EJTGc+He9+1K7ok5Ix0ECcw+/fV6Ct qR9gEmR0jJ5OXzyMyyV4ZQslBZVK261yERwGTa8r1yZW4RJMTGINmpkPv+IoQuvLJ6Nv Q77g== X-Gm-Message-State: AOJu0Yx0UYeoc7ISsmMtzQDtakPCJfISu81PnPFSsViQN6hsrKaH21PB GIhkgf4TE7ZS8Z3JxaXE6Wk5 X-Google-Smtp-Source: AGHT+IGqu3XCBNSXWx1rb5do25rd30Uzw7WzImufME+8fw0H9AfKD11QEb3PPFg58B5Ib+Vce2nB6w== X-Received: by 2002:a05:6830:902:b0:6b8:f588:2c79 with SMTP id v2-20020a056830090200b006b8f5882c79mr4483730ott.1.1697745105671; Thu, 19 Oct 2023 12:51:45 -0700 (PDT) Received: from localhost (pool-68-160-141-91.bstnma.fios.verizon.net. [68.160.141.91]) by smtp.gmail.com with ESMTPSA id h18-20020a0cab12000000b0065afe284b3csm98823qvb.125.2023.10.19.12.51.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Oct 2023 12:51:45 -0700 (PDT) Date: Thu, 19 Oct 2023 15:51:43 -0400 From: Mike Snitzer To: Christian Loehle Cc: agk@redhat.com, dm-devel@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: dm: delay: use kthread instead of timers and wq Message-ID: References: Precedence: bulk X-Mailing-List: dm-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Oct 19 2023 at 7:21P -0400, Christian Loehle wrote: > The current design of timers and wq to realize the delays > is insufficient especially for delays below ~50ms. > The design is replaced with a kthread to flush the expired delays, > trading some CPU time (in some cases) for better delay accuracy and > delays closer to what the user requested for smaller delays. > > Since bios can't be completed in interrupt context using a kthread > is probably the most reasonable way to approach this. Thanks for working on this. The tradeoff to burn cpu to poll for bios that hit their expire time is reasonable for very short delays (which is the focus of your improvement) _but_ for longer delays it is definitely wasteful. Your benchmarks (below) don't quantify the cpu time difference. Might you use a hybrid aproach where over a certain delay threshold it does make sense to avoid waking the polling thread? Otherwise if delay is below the threshold the busy loop is queued immediately? Basically: still use workqueue, but bypass jitter associated with using timer when appropriate (delay is short). There are some inline comments below on your patch. Thanks, Mike > Testing with > echo "0 2097152 zero" | dmsetup create dm-zeros > for i in $(seq 0 20); > do > echo "0 2097152 delay /dev/mapper/dm-zeros 0 $i" | dmsetup create dm-delay-${i}ms; > done > > Some performance numbers for comparison > beaglebone black (single core) CONFIG_HZ_1000=y: > fio --name=1msread --rw=randread --bs=4k --runtime=60 --time_based --filename=/dev/mapper/dm-delay-1ms > Theoretical maximum: 1000 IOPS > Previous: 250 IOPS > Kthread: 500 IOPS > fio --name=10msread --rw=randread --bs=4k --runtime=60 --time_based --filename=/dev/mapper/dm-delay-10ms > Theoretical maximum: 1000 IOPS > Previous: 45 IOPS > Kthread: 50 IOPS > fio --name=1mswrite --rw=randwrite --direct=1 --bs=4k --runtime=60 --time_based --filename=/dev/mapper/dm-delay-1ms > Theoretical maximum: 1000 IOPS > Previous: 498 IOPS > Kthread: 1000 IOPS > fio --name=10mswrite --rw=randwrite --direct=1 --bs=4k --runtime=60 --time_based --filename=/dev/mapper/dm-delay-10ms > Theoretical maximum: 100 IOPS > Previous: 90 IOPS > Kthread: 100 IOPS > fio --name=10mswriteasync --rw=randwrite --direct=1 --bs=4k --runtime=60 --time_based --filename=/dev/mapper/dm-delay-10ms --numjobs=32 --iodepth=64 --ioengine=libaio --group_reporting > Previous: 13.3k IOPS > Kthread: 13.3k IOPS > (This one is just to prove the new design isn't impacting throughput, > not really about delays) > > Signed-off-by: Christian Loehle > --- > drivers/md/dm-delay.c | 83 ++++++++++++------------------------------- > 1 file changed, 22 insertions(+), 61 deletions(-) > > diff --git a/drivers/md/dm-delay.c b/drivers/md/dm-delay.c > index 7433525e5985..a37f28294f7e 100644 > --- a/drivers/md/dm-delay.c > +++ b/drivers/md/dm-delay.c > @@ -13,6 +13,7 @@ > #include > #include > #include > +#include > > #include > > @@ -26,11 +27,9 @@ struct delay_class { > }; > > struct delay_c { > - struct timer_list delay_timer; > - struct mutex timer_lock; > - struct workqueue_struct *kdelayd_wq; > struct work_struct flush_expired_bios; > struct list_head delayed_bios; > + struct task_struct *worker; > atomic_t may_delay; > > struct delay_class read; > @@ -48,42 +47,25 @@ struct dm_delay_info { > }; > > static DEFINE_MUTEX(delayed_bios_lock); > +static void flush_delayed_bios(struct delay_c *dc, int flush_all); > > -static void handle_delayed_timer(struct timer_list *t) > +static int flush_worker_fn(void *data) > { > - struct delay_c *dc = from_timer(dc, t, delay_timer); > + struct delay_c *dc = (struct delay_c *)data; Cast shouldn't be needed. > - queue_work(dc->kdelayd_wq, &dc->flush_expired_bios); > -} > - > -static void queue_timeout(struct delay_c *dc, unsigned long expires) > -{ > - mutex_lock(&dc->timer_lock); > - > - if (!timer_pending(&dc->delay_timer) || expires < dc->delay_timer.expires) > - mod_timer(&dc->delay_timer, expires); > - > - mutex_unlock(&dc->timer_lock); > -} > - > -static void flush_bios(struct bio *bio) > -{ > - struct bio *n; > - > - while (bio) { > - n = bio->bi_next; > - bio->bi_next = NULL; > - dm_submit_bio_remap(bio, NULL); > - bio = n; > + while (1) { > + flush_delayed_bios(dc, 0); > + if (unlikely(list_empty(&dc->delayed_bios))) > + set_current_state(TASK_INTERRUPTIBLE); > + schedule(); cond_resched() likely better? > } > + > + return 0; > } > > -static struct bio *flush_delayed_bios(struct delay_c *dc, int flush_all) > +static void flush_delayed_bios(struct delay_c *dc, int flush_all) Feel free to switch flush_all to a bool. > { > struct dm_delay_info *delayed, *next; > - unsigned long next_expires = 0; > - unsigned long start_timer = 0; > - struct bio_list flush_bios = { }; > > mutex_lock(&delayed_bios_lock); > list_for_each_entry_safe(delayed, next, &dc->delayed_bios, list) { > @@ -91,23 +73,12 @@ static struct bio *flush_delayed_bios(struct delay_c *dc, int flush_all) > struct bio *bio = dm_bio_from_per_bio_data(delayed, > sizeof(struct dm_delay_info)); > list_del(&delayed->list); > - bio_list_add(&flush_bios, bio); > + dm_submit_bio_remap(bio, NULL); > delayed->class->ops--; > - continue; > } > - > - if (!start_timer) { > - start_timer = 1; > - next_expires = delayed->expires; > - } else > - next_expires = min(next_expires, delayed->expires); > } > mutex_unlock(&delayed_bios_lock); > > - if (start_timer) > - queue_timeout(dc, next_expires); > - > - return bio_list_get(&flush_bios); > } > > static void flush_expired_bios(struct work_struct *work) > @@ -115,24 +86,21 @@ static void flush_expired_bios(struct work_struct *work) > struct delay_c *dc; > > dc = container_of(work, struct delay_c, flush_expired_bios); > - flush_bios(flush_delayed_bios(dc, 0)); > + flush_delayed_bios(dc, 0); > } > > static void delay_dtr(struct dm_target *ti) > { > struct delay_c *dc = ti->private; > > - if (dc->kdelayd_wq) > - destroy_workqueue(dc->kdelayd_wq); > - > if (dc->read.dev) > dm_put_device(ti, dc->read.dev); > if (dc->write.dev) > dm_put_device(ti, dc->write.dev); > if (dc->flush.dev) > dm_put_device(ti, dc->flush.dev); > - > - mutex_destroy(&dc->timer_lock); > + if (dc->worker) > + kthread_stop(dc->worker); > > kfree(dc); > } > @@ -188,10 +156,11 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv) > } > > ti->private = dc; > - timer_setup(&dc->delay_timer, handle_delayed_timer, 0); > INIT_WORK(&dc->flush_expired_bios, flush_expired_bios); > INIT_LIST_HEAD(&dc->delayed_bios); > - mutex_init(&dc->timer_lock); > + dc->worker = kthread_create(&flush_worker_fn, dc, "dm-delay-flush-worker"); > + if (!dc->worker) > + goto bad; > atomic_set(&dc->may_delay, 1); > dc->argc = argc; > > @@ -224,13 +193,6 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv) > goto bad; > > out: > - dc->kdelayd_wq = alloc_workqueue("kdelayd", WQ_MEM_RECLAIM, 0); > - if (!dc->kdelayd_wq) { > - ret = -EINVAL; > - DMERR("Couldn't start kdelayd"); > - goto bad; > - } > - > ti->num_flush_bios = 1; > ti->num_discard_bios = 1; > ti->accounts_remapped_io = true; > @@ -260,7 +222,7 @@ static int delay_bio(struct delay_c *dc, struct delay_class *c, struct bio *bio) > list_add_tail(&delayed->list, &dc->delayed_bios); > mutex_unlock(&delayed_bios_lock); > > - queue_timeout(dc, expires); > + wake_up_process(dc->worker); > > return DM_MAPIO_SUBMITTED; > } > @@ -270,8 +232,7 @@ static void delay_presuspend(struct dm_target *ti) > struct delay_c *dc = ti->private; > > atomic_set(&dc->may_delay, 0); > - del_timer_sync(&dc->delay_timer); > - flush_bios(flush_delayed_bios(dc, 1)); > + flush_delayed_bios(dc, 1); > } > > static void delay_resume(struct dm_target *ti) > -- > 2.34.1 >