[PATCH 0/8 v3] bfq: Limit number of allocated scheduler tags per cgroup

Jan Kara
2021-10-06T17:32:04Z

Hello!

Here is the third revision of my patches to fix how bfq weights apply on cgroup
throughput and on throughput of processes with different IO priorities. Since
v2 I've added some more patches so that now IO priorities also result in
service differentiation (previously they had no effect on service
differentiation on some workloads similarly to cgroup weights). The last patch
in the series still needs some work as in the current state it causes a
notable regression (~20-30%) with dbench benchmark for large numbers of
clients. I've verified that the last patch is indeed necessary for the service
differentiation with the workload described in its changelog. As we discussed
with Paolo, I have also found out that if I remove the "waker has enough
budget" condition from bfq_select_queue(), dbench performance is restored
and the service differentiation is still good. But we probably need some
justification or cleaner solution than just removing the condition so that
is still up to discussion. But first seven patches already noticeably improve
the situation for lots of workloads so IMO they stand on their own and
can be merged regardless of how we go about the last patch.

Changes since v2:
* Rebased on top of current Linus' tree
* Updated computation of scheduler tag proportions to work correctly even
for processes within the same cgroup but with different IO priorities
* Added comment roughly explaining why we limit tag depth
* Added patch limiting waker / wakee detection in time so avoid at least the
most obvious false positives
* Added patch to log waker / wakee detections in blktrace for better debugging
* Added patch properly account injected IO

Changes since v1:
* Fixed computation of appropriate proportion of scheduler tags for a cgroup
to work with deeper cgroup hierarchies.

Original cover letter:

I was looking into why cgroup weights do not have any measurable impact on
writeback throughput from different cgroups. This actually a regression from
CFQ where things work more or less OK and weights have roughly the impact they
should. The problem can be reproduced e.g. by running the following easy fio
job in two cgroups with different weight:

[writer]
directory=/mnt/repro/
numjobs=1
rw=write
size=8g
time_based
runtime=30
ramp_time=10
blocksize=1m
direct=0
ioengine=sync

I can observe there's no significat difference in the amount of data written
from different cgroups despite their weights are in say 1:3 ratio.

After some debugging I've understood the dynamics of the system. There are two
issues:

1) The amount of scheduler tags needs to be significantly larger than the
amount of device tags. Otherwise there are not enough requests waiting in BFQ
to be dispatched to the device and thus there's nothing to schedule on.

2) Even with enough scheduler tags, writers from two cgroups eventually start
contending on scheduler tag allocation. These are served on first come first
served basis so writers from both cgroups feed requests into bfq with
approximately the same speed. Since bfq prefers IO from heavier cgroup, that is
submitted and completed faster and eventually we end up in a situation when
there's no IO from the heavier cgroup in bfq and all scheduler tags are
consumed by requests from the lighter cgroup. At that point bfq just dispatches
lots of the IO from the lighter cgroup since there's no contender for disk
throughput. As a result observed throughput for both cgroups are the same.

This series fixes this problem by accounting how many scheduler tags are
allocated for each cgroup and if a cgroup has more tags allocated than its
fair share (based on weights) in its service tree, we heavily limit scheduler
tag bitmap depth for it so that it is not be able to starve other cgroups from
scheduler tags.

What do people think about this?

Honza

Previous versions:
Link: http://lore.kernel.org/r/20210712171146.12231-1-jack@suse.cz # v1
Link: http://lore.kernel.org/r/20210715132047.20874-1-jack@suse.cz # v2