Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On Thu, Jan 18, 2018 at 05:49:10PM -0700, Jens Axboe wrote: > On 1/18/18 5:35 PM, Ming Lei wrote: > > On Thu, Jan 18, 2018 at 05:24:29PM -0700, Jens Axboe wrote: > >> On 1/18/18 5:18 PM, Ming Lei wrote: > >>> On Fri, Jan 19, 2018 at 12:14:24AM +, Bart Van Assche wrote: > On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote: > > On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote: > >> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c > >> index f16096af879a..c59c59cfd2a5 100644 > >> --- a/drivers/md/dm-rq.c > >> +++ b/drivers/md/dm-rq.c > >> @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct > >> blk_mq_hw_ctx *hctx, > >>/* Undo dm_start_request() before requeuing */ > >>rq_end_stats(md, rq); > >>rq_completed(md, rq_data_dir(rq), false); > >> + blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); > >>return BLK_STS_RESOURCE; > >>} > >> > > > > Nak. > > This patch fixes a regression that was introduced by you. You should know > that regressions are not acceptable. If you don't agree with this patch, > please fix the root cause. > >>> > >>> Yesterday I sent a patch, did you test that? > >> > >> That patch isn't going to be of much help. It might prevent you from > >> completely stalling, but it might take you tens of seconds to get there. > > > > Yeah, that is true, and why I marked it as RFC, but the case is so rare to > > happen. > > You don't know that. If the resource is very limited, or someone else > is gobbling up all of it, then it may trigger quite often. Granted, > in that case, you need some way of signaling the freeing of those > resources to make it efficient. > > >> On top of that, it's a rolling timer that gets added when IO is queued, > >> and re-added if IO is pending when it fires. If the latter case is not > >> true, then it won't get armed again. So if IO fails to queue without > >> any other IO pending, you're pretty much in the same situation as if > > > > No IO pending detection may take a bit time, we can do it after > > BLK_STS_RESOURCE is returned and SCHED_RESTART is set. I have done > > this way in the following patch, what do you think of it? > > > > https://github.com/ming1/linux/commit/f866ce0b97b0ae22d033881da7eb07706fd458b4 > > I think it's overly complicated. As I wrote in a lengthier email earlier > today, just ensure that we have a unique return code from the driver for > when it aborts queuing an IO due to a resource shortage that isn't tied > to it's own consumption of it. When we get that return, do a delayed > queue run after X amount of time to ensure we always try again. If you > want to get fancy (later on), you could track the frequency of such > returns and complain if it's too high. Suppose the new introduced return code is BLK_STS_NO_DEV_RESOURCE. This way may degrade performance, for example, DM-MPATH returns BLK_STS_NO_DEV_RESOURCE when blk_get_request() returns NULL, blk-mq handles it by blk_mq_delay_run_hw_queue(10ms). Then blk_mq_sched_restart() is just triggered when one DM-MPATH request is completed, that means one request of underlying queue is completed too, but blk_mq_sched_restart() still can't make progress during the 10ms. That means we should only run blk_mq_delay_run_hw_queue(10ms/100ms) when the queue is idle. I will post out the patch in github for discussion. Thanks, Ming
Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On 1/18/18 5:35 PM, Ming Lei wrote: > On Thu, Jan 18, 2018 at 05:24:29PM -0700, Jens Axboe wrote: >> On 1/18/18 5:18 PM, Ming Lei wrote: >>> On Fri, Jan 19, 2018 at 12:14:24AM +, Bart Van Assche wrote: On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote: > On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote: >> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c >> index f16096af879a..c59c59cfd2a5 100644 >> --- a/drivers/md/dm-rq.c >> +++ b/drivers/md/dm-rq.c >> @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct >> blk_mq_hw_ctx *hctx, >> /* Undo dm_start_request() before requeuing */ >> rq_end_stats(md, rq); >> rq_completed(md, rq_data_dir(rq), false); >> +blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); >> return BLK_STS_RESOURCE; >> } >> > > Nak. This patch fixes a regression that was introduced by you. You should know that regressions are not acceptable. If you don't agree with this patch, please fix the root cause. >>> >>> Yesterday I sent a patch, did you test that? >> >> That patch isn't going to be of much help. It might prevent you from >> completely stalling, but it might take you tens of seconds to get there. > > Yeah, that is true, and why I marked it as RFC, but the case is so rare to > happen. You don't know that. If the resource is very limited, or someone else is gobbling up all of it, then it may trigger quite often. Granted, in that case, you need some way of signaling the freeing of those resources to make it efficient. >> On top of that, it's a rolling timer that gets added when IO is queued, >> and re-added if IO is pending when it fires. If the latter case is not >> true, then it won't get armed again. So if IO fails to queue without >> any other IO pending, you're pretty much in the same situation as if > > No IO pending detection may take a bit time, we can do it after > BLK_STS_RESOURCE is returned and SCHED_RESTART is set. I have done > this way in the following patch, what do you think of it? > > https://github.com/ming1/linux/commit/f866ce0b97b0ae22d033881da7eb07706fd458b4 I think it's overly complicated. As I wrote in a lengthier email earlier today, just ensure that we have a unique return code from the driver for when it aborts queuing an IO due to a resource shortage that isn't tied to it's own consumption of it. When we get that return, do a delayed queue run after X amount of time to ensure we always try again. If you want to get fancy (later on), you could track the frequency of such returns and complain if it's too high. There's no point in adding a lot of code to check whether we need to run the queue or not. Just always do it. We won't be doing it more than 100 times per second for the worst case of the condition taking a while to clear, if we stick to the 10ms re-run time. -- Jens Axboe
Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On 1/18/18 5:18 PM, Ming Lei wrote: > On Fri, Jan 19, 2018 at 12:14:24AM +, Bart Van Assche wrote: >> On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote: >>> On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote: diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c index f16096af879a..c59c59cfd2a5 100644 --- a/drivers/md/dm-rq.c +++ b/drivers/md/dm-rq.c @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx, /* Undo dm_start_request() before requeuing */ rq_end_stats(md, rq); rq_completed(md, rq_data_dir(rq), false); + blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); return BLK_STS_RESOURCE; } >>> >>> Nak. >> >> This patch fixes a regression that was introduced by you. You should know >> that regressions are not acceptable. If you don't agree with this patch, >> please fix the root cause. > > Yesterday I sent a patch, did you test that? That patch isn't going to be of much help. It might prevent you from completely stalling, but it might take you tens of seconds to get there. On top of that, it's a rolling timer that gets added when IO is queued, and re-added if IO is pending when it fires. If the latter case is not true, then it won't get armed again. So if IO fails to queue without any other IO pending, you're pretty much in the same situation as if you marked the queue as restart. Nobody is going to be noticing either of those conditions. -- Jens Axboe
Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On Fri, 2018-01-19 at 08:18 +0800, Ming Lei wrote: > On Fri, Jan 19, 2018 at 12:14:24AM +, Bart Van Assche wrote: > > On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote: > > > On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote: > > > > diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c > > > > index f16096af879a..c59c59cfd2a5 100644 > > > > --- a/drivers/md/dm-rq.c > > > > +++ b/drivers/md/dm-rq.c > > > > @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct > > > > blk_mq_hw_ctx *hctx, > > > > /* Undo dm_start_request() before requeuing */ > > > > rq_end_stats(md, rq); > > > > rq_completed(md, rq_data_dir(rq), false); > > > > + blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); > > > > return BLK_STS_RESOURCE; > > > > } > > > > > > > > > > Nak. > > > > This patch fixes a regression that was introduced by you. You should know > > that regressions are not acceptable. If you don't agree with this patch, > > please fix the root cause. > > Yesterday I sent a patch, did you test that? Yes, I did. It caused queue stalls that were so bad that sending "kick" to the debugfs "state" attribute was not sufficient to resolve the queue stall. Bart.
Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On Fri, Jan 19, 2018 at 12:14:24AM +, Bart Van Assche wrote: > On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote: > > On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote: > > > diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c > > > index f16096af879a..c59c59cfd2a5 100644 > > > --- a/drivers/md/dm-rq.c > > > +++ b/drivers/md/dm-rq.c > > > @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct > > > blk_mq_hw_ctx *hctx, > > > /* Undo dm_start_request() before requeuing */ > > > rq_end_stats(md, rq); > > > rq_completed(md, rq_data_dir(rq), false); > > > + blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); > > > return BLK_STS_RESOURCE; > > > } > > > > > > > Nak. > > This patch fixes a regression that was introduced by you. You should know > that regressions are not acceptable. If you don't agree with this patch, > please fix the root cause. Yesterday I sent a patch, did you test that? -- Ming
Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote: > On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote: > > diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c > > index f16096af879a..c59c59cfd2a5 100644 > > --- a/drivers/md/dm-rq.c > > +++ b/drivers/md/dm-rq.c > > @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx > > *hctx, > > /* Undo dm_start_request() before requeuing */ > > rq_end_stats(md, rq); > > rq_completed(md, rq_data_dir(rq), false); > > + blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); > > return BLK_STS_RESOURCE; > > } > > > > Nak. This patch fixes a regression that was introduced by you. You should know that regressions are not acceptable. If you don't agree with this patch, please fix the root cause. Bart.
Re: [PATCH] dm rq: Avoid that request processing stalls sporadically
On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote:
> If the .queue_rq() implementation of a block driver returns
> BLK_STS_RESOURCE then that block driver is responsible for
> rerunning the queue once the condition that caused it to return
> BLK_STS_RESOURCE has been cleared. The dm-mpath driver tells the
> dm core to requeue a request if e.g. not enough memory is
> available for cloning a request or if the underlying path is
> busy. Since the dm-mpath driver does not receive any kind of
> notification if the condition that caused it to return "requeue"
> is cleared, the only solution to avoid that dm-mpath request
> processing stalls is to call blk_mq_delay_run_hw_queue(). Hence
> this patch.
>
> Fixes: ec3eaf9a6731 ("dm mpath: don't call blk_mq_delay_run_hw_queue() in
> case of BLK_STS_RESOURCE")
> Signed-off-by: Bart Van Assche
> Cc: Ming Lei
> ---
> drivers/md/dm-rq.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
> index f16096af879a..c59c59cfd2a5 100644
> --- a/drivers/md/dm-rq.c
> +++ b/drivers/md/dm-rq.c
> @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx
> *hctx,
> /* Undo dm_start_request() before requeuing */
> rq_end_stats(md, rq);
> rq_completed(md, rq_data_dir(rq), false);
> + blk_mq_delay_run_hw_queue(hctx, 100/*ms*/);
> return BLK_STS_RESOURCE;
> }
>
Nak.
It still takes a bit time to add this request to hctx->dispatch_list
from here, so suppose the time is longer than 100ms because of interrupt
, preemption or whatever, this request can't be observed in the scheduled
run queue(__blk_mq_run_hw_queue).
Not mention it is just a ugly workaround, which degrades performance
a lot.
--
Ming
[PATCH] dm rq: Avoid that request processing stalls sporadically
If the .queue_rq() implementation of a block driver returns
BLK_STS_RESOURCE then that block driver is responsible for
rerunning the queue once the condition that caused it to return
BLK_STS_RESOURCE has been cleared. The dm-mpath driver tells the
dm core to requeue a request if e.g. not enough memory is
available for cloning a request or if the underlying path is
busy. Since the dm-mpath driver does not receive any kind of
notification if the condition that caused it to return "requeue"
is cleared, the only solution to avoid that dm-mpath request
processing stalls is to call blk_mq_delay_run_hw_queue(). Hence
this patch.
Fixes: ec3eaf9a6731 ("dm mpath: don't call blk_mq_delay_run_hw_queue() in case
of BLK_STS_RESOURCE")
Signed-off-by: Bart Van Assche
Cc: Ming Lei
---
drivers/md/dm-rq.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index f16096af879a..c59c59cfd2a5 100644
--- a/drivers/md/dm-rq.c
+++ b/drivers/md/dm-rq.c
@@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx
*hctx,
/* Undo dm_start_request() before requeuing */
rq_end_stats(md, rq);
rq_completed(md, rq_data_dir(rq), false);
+ blk_mq_delay_run_hw_queue(hctx, 100/*ms*/);
return BLK_STS_RESOURCE;
}
--
2.15.1
