Re: [RFC PATCH 0/6] Drain remote per-cpu directly v2

[Minchan Kim <minchan@kernel.org>]

On Tue, May 10, 2022 at 10:27:33AM +0100, Mel Gorman wrote:
> On Mon, May 09, 2022 at 08:58:51AM -0700, Minchan Kim wrote:
> > On Mon, May 09, 2022 at 02:07:59PM +0100, Mel Gorman wrote:
> > > Changelog since v1
> > > o Fix unsafe RT locking scheme
> > > o Use spin_trylock on UP PREEMPT_RT
> > 
> > Mel,
> > 
> > 
> > Is this only change from previous last version which has some
> > delta you fixed based on Vlastimil and me?
> > 
> 
> Full diff is below although it can also be generated by
> comparing the mm-pcpdrain-v1r8..mm-pcpdrain-v2r1 branches in
> https://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git/

I took the delta when I started testing so the testing result
would be valid. Thanks.

> 
> > And is it still RFC?
> > 
> 
> It's still RFC because it's a different approach to Nicolas' series and
> I want at least his Acked-by before taking the RFC label out and sending
> it to Andrew.
> 
> > Do you have some benchmark data?
> > 
> 
> Yes, but as reclaim is not fundamentally altered the main difference
> in behavious is that work is done inline instead of being deferred to a
> workqueue. That means in some cases, system CPU usage of a task will be
> higher because it's paying the cost directly.

Sure but the reclaim path is already expensive so I doubt we could
see the sizable measurement on the system CPU usage.

What I wanted to see was whether we have regression due to adding
spin_lock/unlock instructions in hot path. Due to squeeze it to
a cacheline, I expected the regression would be just marginal.

> 
> The workloads I used just hit reclaim directly to make sure it's
> functionally not broken. There is no change in page aging decisions,
> only timing of drains. I didn't check interference of a heavy workload
> interfering with a CPU-bound workload running on NOHZ CPUs as I assumed
> both you and Nicolas had a test case ready to use.

The my workload is not NOHZ CPUs but run apps under heavy memory
pressure so they goes to direct reclaim and be stuck on drain_all_pages
until work on workqueue run.

unit: nanosecond
max(dur)        avg(dur)                count(dur)
166713013       487511.77786438033      1283

From traces, system encountered the drain_all_pages 1283 times and
worst case was 166ms and avg was 487us.

The other problem was alloc_contig_range in CMA. The PCP draining
takes several hundred millisecond sometimes though there is no
memory pressure or a few of pages to be migrated out but CPU were
fully booked.

Your patch perfectly removed those wasted time.

> 
> The main one I paid interest to was a fault latency benchmark in
> the presense of heavy reclaim called stutterp. It simulates a simple
> workload. One part uses a lot of anonymous memory, a second measures mmap
> latency and a third copies a large file.  The primary metric is checking
> for mmap latency. It was originally put together to debug interactivity
> issues on a desktop in the presense of heavy IO where the desktop
> applications were being pushed to backing storage.
> 
> stutterp
>                               5.18.0-rc1             5.18.0-rc1
>                                  vanilla       mm-pcpdrain-v2r1
> 1st-qrtle mmap-4      15.9557 (   0.00%)     15.4045 (   3.45%)
> 1st-qrtle mmap-6      10.8025 (   0.00%)     11.1204 (  -2.94%)
> 1st-qrtle mmap-8      16.9338 (   0.00%)     17.0595 (  -0.74%)
> 1st-qrtle mmap-12     41.4746 (   0.00%)      9.4003 (  77.33%)
> 1st-qrtle mmap-18     47.7123 (   0.00%)    100.0275 (-109.65%)
> 1st-qrtle mmap-24     17.7098 (   0.00%)     16.9633 (   4.22%)
> 1st-qrtle mmap-30     69.2565 (   0.00%)     38.2205 (  44.81%)
> 1st-qrtle mmap-32     49.1295 (   0.00%)     46.8819 (   4.57%)
> 3rd-qrtle mmap-4      18.4706 (   0.00%)     17.4799 (   5.36%)
> 3rd-qrtle mmap-6      11.4526 (   0.00%)     11.5567 (  -0.91%)
> 3rd-qrtle mmap-8      19.5903 (   0.00%)     19.0046 (   2.99%)
> 3rd-qrtle mmap-12     50.3095 (   0.00%)     25.3254 (  49.66%)
> 3rd-qrtle mmap-18     67.3319 (   0.00%)    147.6404 (-119.27%)
> 3rd-qrtle mmap-24     41.3779 (   0.00%)     84.4035 (-103.98%)
> 3rd-qrtle mmap-30    127.1375 (   0.00%)    148.8884 ( -17.11%)
> 3rd-qrtle mmap-32     79.7423 (   0.00%)    182.3042 (-128.62%)
> Max-99    mmap-4      46.9123 (   0.00%)     49.7731 (  -6.10%)
> Max-99    mmap-6      42.5414 (   0.00%)     16.6173 (  60.94%)
> Max-99    mmap-8      43.1237 (   0.00%)     23.3478 (  45.86%)
> Max-99    mmap-12     62.8025 (   0.00%)   1947.3862 (-3000.81%)
> Max-99    mmap-18  27936.8695 (   0.00%)    232.7122 (  99.17%)
> Max-99    mmap-24 204543.9436 (   0.00%)   5805.2478 (  97.16%)
> Max-99    mmap-30   2350.0289 (   0.00%)  10300.6344 (-338.32%)
> Max-99    mmap-32  56164.2271 (   0.00%)   7789.7526 (  86.13%)
> Max       mmap-4     840.3468 (   0.00%)   1137.4462 ( -35.35%)
> Max       mmap-6  255233.3996 (   0.00%)  91304.0952 (  64.23%)
> Max       mmap-8  210910.6497 (   0.00%) 117931.0796 (  44.08%)
> Max       mmap-12 108268.9537 (   0.00%) 319971.6910 (-195.53%)
> Max       mmap-18 608805.3195 (   0.00%) 197483.2205 (  67.56%)
> Max       mmap-24 327697.5605 (   0.00%) 382842.5356 ( -16.83%)
> Max       mmap-30 688684.5335 (   0.00%) 669992.7705 (   2.71%)
> Max       mmap-32 396842.0114 (   0.00%) 415978.2539 (  -4.82%)
> 
>                   5.18.0-rc1  5.18.0-rc1
>                      vanillamm-pcpdrain-v2r1
> Duration User        1438.08     1637.21
> Duration System     12267.41    10307.96
> Duration Elapsed     3929.70     3443.53
> 
> 
> It's a mixed bag but this workload is always a mixed bag and it's stressing
> reclaim.  At some points, latencies are worse, in others better. Overall,
> it completed faster and this was on a 1-socket machine.
> 
> On a 2-socket machine, the overall completions times were worse
> 
>                   5.18.0-rc1  5.18.0-rc1
>                      vanillamm-pcpdrain-v2r1
> Duration User        3713.75     2899.90
> Duration System    303507.56   378909.94
> Duration Elapsed    15444.59    19067.40
> 
> In general this type of workload is variable given the nature of what it
> does and can give different results on each execution. When originally
> designed, it was to deal with stalls lasting several seconds to reduce
> them to the sub-millisecond range.
> 
> The intent of the series is switching out-of-line work to in-line so
> what it should be measuring is interference effects and not straight-line
> performance and I haven't written a specific test case yet. When writing
> the series initially, it was to investigate if the PCP could be lockless
> and failing that, if disabling IRQs could be avoided in the common case.
> It just turned out that part of that made remote draining possible and
> I focused closer on that because it's more important.
> 
> > I'd like to give Acked-by/Tested-by(even though there are a few
> > more places to align with new fields name in 1/6)
> 
> Which ones are of concern?
> 
> Some of the page->lru references I left alone in the init paths simply
> because in those contexts, the page wasn't on a buddy or PCP list. In
> free_unref_page_list the page is not on the LRU, it's just been isolated
> from the LRU. In alloc_pages_bulk, it's not on a buddy, pcp or LRU list
> and is just a list placeholder so I left it alone. In
> free_tail_pages_check the context was a page that was likely previously
> on a LRU.

Just nits: all are list macros.

free_pcppages_bulk's list_last_entry should be pcp_list.

mark_free_pages's list_for_each_entry should be buddy_list

__rmqueue_pcplist's list_first_enty should be pcp_list.

> 
> > since I have
> > tested these patchset in my workload and didn't spot any other
> > problems.
> > 
> 
> Can you describe this workload, is it available anywhere and does it
> require Android to execute?

I wrote down above. It runs on Android but I don't think it's
android specific issue but anyone could see such a long latency
from PCP draining once one of cores are monopolized by higher
priority processes or too many pending kworks.

> 
> If you have positive results, it would be appreciated if you could post
> them or just note in a Tested-by/Acked-by that it had a measurable impact
> on the reclaim/cma path.

Sure.

All patches in this series.

Tested-by: Minchan Kim <minchan@kernel.org>
Acked-by: Minchan Kim <minchan@kernel.org>

Thanks.