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

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, May 10, 2022 at 11:13:05AM -0700, Minchan Kim wrote:
> > 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.
> 

It would be difficult to distinguish from the noise.

> 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.
> 

Ah, yes, I did test for this. page-fault-test hits the relevant paths
very heavily and did show minor differences.

                                     5.18.0-rc1               5.18.0-rc1
                                        vanilla         mm-pcpdrain-v2r1
Hmean     faults/sec-1   886331.5718 (   0.00%)   885462.7479 (  -0.10%)
Hmean     faults/sec-3  2337706.1583 (   0.00%)  2332130.4909 *  -0.24%*
Hmean     faults/sec-5  2851594.2897 (   0.00%)  2844123.9307 (  -0.26%)
Hmean     faults/sec-7  3543251.5507 (   0.00%)  3516889.0442 *  -0.74%*
Hmean     faults/sec-8  3947098.0024 (   0.00%)  3916162.8476 *  -0.78%*
Stddev    faults/sec-1     2302.9105 (   0.00%)     2065.0845 (  10.33%)
Stddev    faults/sec-3     7275.2442 (   0.00%)     6033.2620 (  17.07%)
Stddev    faults/sec-5    24726.0328 (   0.00%)    12525.1026 (  49.34%)
Stddev    faults/sec-7     9974.2542 (   0.00%)     9543.9627 (   4.31%)
Stddev    faults/sec-8     9468.0191 (   0.00%)     7958.2607 (  15.95%)
CoeffVar  faults/sec-1        0.2598 (   0.00%)        0.2332 (  10.24%)
CoeffVar  faults/sec-3        0.3112 (   0.00%)        0.2587 (  16.87%)
CoeffVar  faults/sec-5        0.8670 (   0.00%)        0.4404 (  49.21%)
CoeffVar  faults/sec-7        0.2815 (   0.00%)        0.2714 (   3.60%)
CoeffVar  faults/sec-8        0.2399 (   0.00%)        0.2032 (  15.28%)

There is a small hit in the number of faults per second but it's within
the noise and the results are more stable with the series so I'd mark it
down as a small but potentially measurable impact.

> > 
> > 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.
> 

Those stalls are painful and it's a direct impact where a workload does
not make progress. The NOHZ stall is different in that it's worried
about interference. Both problems should have the same solution.

Do you mind if I quote these paragraphs in the leader to v3?

> > 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.
> 

Ah, you're completely correct.

> > 
> > > 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.
> 

Yeah, I agree it's not an Android-specific problem. It could be detected by
tracing the time spent in drain_all_pages for any arbitrary workload. The
BCC funclatency tool could measure it.

> > 
> > 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, I've added that to all the patches. I'll wait another day for
more feedback before sending out a v3. The following is the diff between
v2 and v3 based on your feedback.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4ac39d30ec8f..0f5a6a5b0302 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1497,7 +1497,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 		do {
 			int mt;
 
-			page = list_last_entry(list, struct page, lru);
+			page = list_last_entry(list, struct page, pcp_list);
 			mt = get_pcppage_migratetype(page);
 
 			/* must delete to avoid corrupting pcp list */
@@ -3276,7 +3276,7 @@ void mark_free_pages(struct zone *zone)
 
 	for_each_migratetype_order(order, t) {
 		list_for_each_entry(page,
-				&zone->free_area[order].free_list[t], lru) {
+				&zone->free_area[order].free_list[t], buddy_list) {
 			unsigned long i;
 
 			pfn = page_to_pfn(page);
@@ -3761,7 +3761,7 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
 			}
 		}
 
-		page = list_first_entry(list, struct page, lru);
+		page = list_first_entry(list, struct page, pcp_list);
 		list_del(&page->pcp_list);
 		pcp->count -= 1 << order;
 	} while (check_new_pcp(page, order));

-- 
Mel Gorman
SUSE Labs