Re: blk-throttle.c : When limit is changed, must start a new slice

[Vivek Goyal]

On Tue, Mar 08, 2011 at 10:33:47PM +0800, lina wrote:
> >On Mon, Mar 07, 2011 at 11:05:49PM +0800, lina wrote:
> >> >On Sat, Mar 05, 2011 at 12:30:55AM +0800, lina wrote:
> >> >> Hi Vivek,    
> >> >>      Take you a litter time to read this letter, I think there seens  to 
> >> >>  be a small bug in blk-throttle.c.
> >> >>      This might happen that initially cgroup limit was greater than the 
> >> >>  device's physic ability, but later limit was under physic ability. In this
> >> >>  case, all of the bio in the device was throttled for serveral minutes. 
> >> >>   
> >> >>      I did some analysis as the following:
> >> >>      First, setting a very large bps on device lead all of the bio go
> >> >>  through. The slice is begin when test begin, and only extend but 
> >> >>  can not start new one. 
> >> >>      Second, change the limit under physic ability to make one bio 
> >> >>  queued. Once one bio queued, blk_throtl_bio func will call 
> >> >>  tg_update_disptime to estimate the delay time for the throtl_work.
> >> >>      As the slice is very old, there is a very large value in 
> >> >>  tg->bytes_disp[rw], and the tg->disptime is a long time after jiffies. 
> >> >>  During this time, all of the bio is queued. And the work_queue can 
> >> >>  not start, so tg->slice_start[rw] still can not be reset.
> >> >>   
> >> >>      Although after serveral minutes everything will be ok, but it still
> >> >>  seens no-good for users.
> >> >>      
> >> >>      I think it should start a new slice when the limit is changed. 
> >> >>      Here is my patch, please conrrect it if there is something wrong
> >> >>  follow.
> >>  >
> >> >
> >> >CCing to lkml. Lets keep all the testing and bug reports regarding
> >> >blkio throttling on mailing list.
> >> >
> >> >thanks for the bug report lina. I think this is a bug. I am not too keen
> >> >on restarting slice all the time upon limit change as somebody can exploit
> >> >that to get higher BW by doing it frequently. Can you try attached patch
> >> >and see if it solves your problem.
> >> 
> >>  Thank you for this patch, it can solve the problem. But there still has 5~10
> >>  seconds 0 bps in the test. I think this is because we first tirm the end of
> >>  slice, then new one, there is some latency. Do you have any idea to let the 
> >>  limit change work immediately or make less latency(maybe 1 or 2 seconds
> >>   
> >
> >Ok, if you are concerned about those few seconds, can you please try
> >following patch. I think starting a new slice is better when processing
> >limit change instead of in blk_throtl_bio().
> >
>   
>  Yes. It's better starting a new slice in processing limit change function. I
>  have test starting a new slice in blk_throtl_bio(), if I change the limit very
>  quickly, the bps go out of my control.

I thought about changing limits too quickly. Is this a problem? In many
setups a non-priviliged user will not be able to write to limit files so
he can not exploit it. root should not anyway do that. To get higher
bandwidth root can simply write to files and there is no reason to try
to write to reset limit frequently.

So resetting the slice upon limit change probably is not a huge problem then.

>   
>  Unfortunately, the following patch still has 5~10 seconds latency. I have no
>  idea to resolve this problem, it seens hard to find a more suitable func to
>  call throtl_start_new_slice().

So are you saying that following patch did not solve the latnecy issue?
Resetting slice upon limit change did not work for you?

Thanks
Vivek

> >
> >---
> > block/blk-throttle.c |   21 +++++++++++++++++++++
> > 1 file changed, 21 insertions(+)
> >
> >Index: linux-2.6/block/blk-throttle.c
> >===================================================================
> >--- linux-2.6.orig/block/blk-throttle.c 2011-03-04 13:59:45.000000000 -0500
> >+++ linux-2.6/block/blk-throttle.c 2011-03-07 10:54:30.639467538 -0500
> >@@ -757,6 +757,14 @@ static void throtl_process_limit_change(
> >  " riops=%u wiops=%u", tg->bps[READ],
> >  tg->bps[WRITE], tg->iops[READ],
> >  tg->iops[WRITE]);
> >+ /*
> >+ * Restart the slices for both READ and WRITES. It
> >+ * might happen that a group's limit are dropped
> >+ * suddenly and we don't want to account recently
> >+ * dispatched IO with new low rate
> >+ */
> >+ throtl_start_new_slice(td, tg, 0);
> >+ throtl_start_new_slice(td, tg, 1);
> >  tg_update_disptime(td, tg);
> >  tg->limits_changed = false;
> >  }
> >@@ -1023,6 +1031,19 @@ int blk_throtl_bio(struct request_queue 
> >  /* Bio is with-in rate limit of group */
> >  if (tg_may_dispatch(td, tg, bio, NULL)) {
> >  throtl_charge_bio(tg, bio);
> >+
> >+ /*
> >+ * We need to trim slice even when bios are not being queued
> >+ * otherwise it might happen that a bio is not queued for
> >+ * a long time and slice keeps on extending and trim is not
> >+ * called for a long time. Now if limits are reduced suddenly
> >+ * we take into account all the IO dispatched so far at new
> >+ * low rate and * newly queued IO gets a really long dispatch
> >+ * time.
> >+ *
> >+ * So keep on trimming slice even if bio is not queued.
> >+ */
> >+ throtl_trim_slice(td, tg, rw);
> >  goto out;
> >  }
>  
> Thanks
>  Lina
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