Re: [PATCH] block: fix -EAGAIN IOPOLL task/vm accounting

[Ming Lei <ming.lei@redhat.com>]

On Sun, Aug 30, 2020 at 04:28:00PM +0100, Christoph Hellwig wrote:
> On Sun, Aug 30, 2020 at 09:09:02AM -0600, Jens Axboe wrote:
> > On 8/30/20 12:26 AM, Christoph Hellwig wrote:
> > > On Sat, Aug 29, 2020 at 10:51:11AM -0600, Jens Axboe wrote:
> > >> We currently increment the task/vm counts when we first attempt to queue a
> > >> bio. But this isn't necessarily correct - if the request allocation fails
> > >> with -EAGAIN, for example, and the caller retries, then we'll over-account
> > >> by as many retries as are done.
> > >>
> > >> This can happen for polled IO, where we cannot wait for requests. Hence
> > >> retries can get aggressive, if we're running out of requests. If this
> > >> happens, then watching the IO rates in vmstat are incorrect as they count
> > >> every issue attempt as successful and hence the stats are inflated by
> > >> quite a lot potentially.
> > >>
> > >> Add a bio flag to know if we've done accounting or not. This prevents
> > >> the same bio from being accounted potentially many times, when retried.
> > > 
> > > Can't the resubmitter just use submit_bio_noacct?  What is the call
> > > stack here?
> > 
> > The resubmitter is way higher than that. You could potentially have that
> > done in the block layer, but not higher up.
> > 
> > The use case is async submissions, going through ->read_iter() again.
> > Or ->write_iter().
> 
> But how does a bio flag help there?  If we go through the file ops
> again the next submission will be a new bio structure.

Yeah, we also have use cases of stack bio variable.



Thanks,
Ming