Re: False waker detection in BFQ

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 13 ago 2021, alle ore 16:01, Jan Kara <jack@suse.cz> ha scritto:
> 
> Hi Paolo!
> 
> On Thu 20-05-21 17:05:45, Paolo Valente wrote:
>>> Il giorno 5 mag 2021, alle ore 18:20, Jan Kara <jack@suse.cz> ha scritto:
>>> 
>>> Hi Paolo!
>>> 
>>> I have two processes doing direct IO writes like:
>>> 
>>> dd if=/dev/zero of=/mnt/file$i bs=128k oflag=direct count=4000M
>>> 
>>> Now each of these processes belongs to a different cgroup and it has
>>> different bfq.weight. I was looking into why these processes do not split
>>> bandwidth according to BFQ weights. Or actually the bandwidth is split
>>> accordingly initially but eventually degrades into 50/50 split. After some
>>> debugging I've found out that due to luck, one of the processes is decided
>>> to be a waker of the other process and at that point we loose isolation
>>> between the two cgroups. This pretty reliably happens sometime during the
>>> run of these two processes on my test VM. So can we tweak the waker logic
>>> to reduce the chances for false positives? Essentially when there are only
>>> two processes doing heavy IO against the device, the logic in
>>> bfq_check_waker() is such that they are very likely to eventually become
>>> wakers of one another. AFAICT the only condition that needs to get
>>> fulfilled is that they need to submit IO within 4 ms of the completion of
>>> IO of the other process 3 times.
>>> 
>> 
>> Hi Jan!
>> as I happened to tell you moths ago, I feared some likely cover case
>> to show up eventually.  Actually, I was even more pessimistic than how
>> reality proved to be :)
>> 
>> I'm sorry for my delay, but I've had to think about this issue for a
>> while.  Being too strict would easily run out journald as a waker for
>> processes belonging to a different group.
>> 
>> So, what do you think of this proposal: add the extra filter that a
>> waker must belong to the same group of the woken, or, at most, to the
>> root group?
> 
> Returning back to this :). I've been debugging other BFQ problems with IO
> priorities not really leading to service differentiation (mostly because
> scheduler tag exhaustion, false waker detection, and how we inject IO for a
> waker) and as a result I have come up with a couple of patches that also
> address this issue as a side effect - I've added an upper time limit
> (128*slice_idle) for the "third cooperation" detection and that mostly got
> rid of these false waker detections.

Great!

> We could fail to detect waker-wakee
> processes if they do not cooperate frequently but then the value of the
> detection is small and the lack of isolation may do more harm than good
> anyway.
> 

IIRC, dbench was our best benchmark for checking whether the detection is
(still) effective.


> Currently I'm running wider set of benchmarks for the patches to see
> whether I didn't regress anything else. If not, I'll post the patches to
> the list.
> 

Any news?

Thanks,
Paolo

> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR