On Fri, Oct 18, 2019 at 03:01:27PM +0200, Michal Hocko wrote:
> On Fri 18-10-19 11:56:05, Mel Gorman wrote:
> > Deferred memory initialisation updates zone->managed_pages during
> > the initialisation phase but before that finishes, the per-cpu page
> > allocator (pcpu) calculates the number of pages allocated/freed in
> > batches as well as the maximum number of pages allowed on a per-cpu list.
> > As zone->managed_pages is not up to date yet, the pcpu initialisation
> > calculates inappropriately low batch and high values.
> >
> > This increases zone lock contention quite severely in some cases with the
> > degree of severity depending on how many CPUs share a local zone and the
> > size of the zone. A private report indicated that kernel build times were
> > excessive with extremely high system CPU usage. A perf profile indicated
> > that a large chunk of time was lost on zone->lock contention.
> >
> > This patch recalculates the pcpu batch and high values after deferred
> > initialisation completes on each node. It was tested on a 2-socket AMD
> > EPYC 2 machine using a kernel compilation workload -- allmodconfig and
> > all available CPUs.
> >
> > mmtests configuration: config-workload-kernbench-max
> > Configuration was modified to build on a fresh XFS partition.
> >
> > kernbench
> > 5.4.0-rc3 5.4.0-rc3
> > vanilla resetpcpu-v1r1
> > Amean user-256 13249.50 ( 0.00%) 15928.40 * -20.22%*
> > Amean syst-256 14760.30 ( 0.00%) 4551.77 * 69.16%*
> > Amean elsp-256 162.42 ( 0.00%) 118.46 * 27.06%*
> > Stddev user-256 42.97 ( 0.00%) 50.83 ( -18.30%)
> > Stddev syst-256 336.87 ( 0.00%) 33.70 ( 90.00%)
> > Stddev elsp-256 2.46 ( 0.00%) 0.81 ( 67.01%)
> >
> > 5.4.0-rc3 5.4.0-rc3
> > vanillaresetpcpu-v1r1
> > Duration User 39766.24 47802.92
> > Duration System 44298.10 13671.93
> > Duration Elapsed 519.11 387.65
> >
> > The patch reduces system CPU usage by 69.16% and total build time by
> > 27.06%. The variance of system CPU usage is also much reduced.
>
> The fix makes sense. It would be nice to see the difference in the batch
> sizes from the initial setup compared to the one after the deferred
> intialization is done
>
Before, this was the breakdown of batch and high values over all zones
were
256 batch: 1
256 batch: 63
512 batch: 7
256 high: 0
256 high: 378
512 high: 42
i.e. 512 pcpu pagesets had a batch limit of 7 and a high limit of 42.
These were for the NORMAL zones on the system. After the patch
256 batch: 1
768 batch: 63
256 high: 0
768 high: 378
> > Cc: stable@xxxxxxxxxxxxxxx # v4.15+
>
> Hmm, are you sure about 4.15? Doesn't this go all the way down to
> deferred initialization? I do not see any recent changes on when
> setup_per_cpu_pageset is called.
>
No, I'm not 100% sure. It looks like this was always an issue from the
code but did not happen on at least one 4.12-based distribution kernel for
reasons that are non-obvious. Either way, the tag should have been "v4.1+"
Thanks.
--
Mel Gorman
SUSE Labs
