Re: [PATCH v5 0/2] fix inaccurate io_ticks

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

On Tue, Nov 17, 2020 at 12:59:46PM +0800, Weiping Zhang wrote:
> On Tue, Nov 17, 2020 at 11:28 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Tue, Nov 17, 2020 at 11:01:49AM +0800, Weiping Zhang wrote:
> > > Hi Jens,
> > >
> > > Ping
> >
> > Hello Weiping,
> >
> > Not sure we have to fix this issue, and adding blk_mq_queue_inflight()
> > back to IO path brings cost which turns out to be visible, and I did
> > get soft lockup report on Azure NVMe because of this kind of cost.
> >
> Have you test v5, this patch is different from v1, the v1 gets
> inflight for each IO,
> v5 has changed to get inflight every jiffer.

I meant the issue can be reproduced on kernel before 5b18b5a73760("block:
delete part_round_stats and switch to less precise counting").

Also do we really need to fix this issue? I understand device
utilization becomes not accurate at very small load, is it really
worth of adding runtime load in fast path for fixing this issue?

> 
> If for v5, can we reproduce it on null_blk ?

No, I just saw report on Azure NVMe.

> 
> > BTW, suppose the io accounting issue needs to be fixed, just wondering
> > why not simply revert 5b18b5a73760 ("block: delete part_round_stats and
> > switch to less precise counting"), and the original way had been worked
> > for decades.
> >
> This patch is more better than before, it will break early when find there is
> inflight io on any cpu, for the worst case(the io in running on the last cpu),
> it iterates all cpus.

Please see the following case:

1) one device has 256 hw queues, and the system has 256 cpu cores, and
each hw queue's depth is 1k.

2) there isn't any io load on CPUs(0 ~ 254)

3) heavy io load is run on CPU 255

So with your trick the code still need to iterate hw queues from 0 to 254, and
the load isn't something which can be ignored. Especially it is just for
io accounting.


Thanks,
Ming