Subject: Re: [1/1] Block device throttling [Re: Distributed storage.]

From: Daniel Phillips <[email-masked]>
Date: Thu, 30 Aug 2007 16:20:35 -0700

On Wednesday 29 August 2007 01:53, Evgeniy Polyakov wrote:
> Then, if of course you will want, which I doubt, you can reread
> previous mails and find that it was pointed to that race and
> possibilities to solve it way too long ago.

What still bothers me about your response is that, while you know the 
race exists and do not disagree with my example, you don't seem to see 
that that race can eventually lock up the block device by repeatedly 
losing throttle counts which are never recovered.  What prevents that?

> > --- 2.6.22.clean/block/ll_rw_blk.c	2007-07-08 16:32:17.000000000
> > -0700 +++ 2.6.22/block/ll_rw_blk.c	2007-08-24 12:07:16.000000000
> > -0700 @@ -3237,6 +3237,15 @@ end_io:
> >   */
> >  void generic_make_request(struct bio *bio)
> >  {
> > +	struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> > +
> > +	if (q && q->metric) {
> > +		int need = bio->bi_reserved = q->metric(bio);
> > +		bio->queue = q;
>
> In case you have stacked device, this entry will be rewritten and you
> will lost all your account data.

It is a weakness all right.  Well,

-	if (q && q->metric) {
+	if (q && q->metric && !bio->queue) {

which fixes that problem.  Maybe there is a better fix possible.  Thanks 
for the catch!

The original conception was that this block throttling would apply only 
to the highest level submission of the bio, the one that crosses the 
boundary between filesystem (or direct block device application) and 
block layer.  Resubmitting a bio or submitting a dependent bio from 
inside a block driver does not need to be throttled because all 
resources required to guarantee completion must have been obtained 
_before_ the bio was allowed to proceed into the block layer.

The other principle we are trying to satisfy is that the throttling 
should not be released until bio->endio, which I am not completely sure 
about with the patch as modified above.  Your earlier idea of having 
the throttle protection only cover the actual bio submission is 
interesting and may be effective in some cases, in fact it may cover 
the specific case of ddsnap.  But we don't have to look any further 
than ddraid (distributed raid) to find a case it doesn't cover - the 
additional memory allocated to hold parity data has to be reserved 
until parity data is deallocated, long after the submission completes.
So while you manage to avoid some logistical difficulties, it also looks 
like you didn't solve the general problem.

Hopefully I will be able to report on whether my patch actually works 
soon, when I get back from vacation.  The mechanism in ddsnap this is 
supposed to replace is effective, it is just ugly and tricky to verify.

Regards,

Daniel