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Wed, 17 Aug 2022 15:17:01 -0400 X-MC-Unique: KMGSa4NXMNaSg14xOG0Htw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id BFBA73C10149; Wed, 17 Aug 2022 19:17:00 +0000 (UTC) Received: from fuller.cnet (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6B1701121315; Wed, 17 Aug 2022 19:17:00 +0000 (UTC) Received: by fuller.cnet (Postfix, from userid 1000) id 522CB416D5DE; Wed, 17 Aug 2022 16:16:24 -0300 (-03) Message-ID: <20220817191346.287594886@redhat.com> User-Agent: quilt/0.66 Date: Wed, 17 Aug 2022 16:13:46 -0300 From: Marcelo Tosatti To: atomlin@redhat.com, frederic@kernel.org Cc: cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v7 0/3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 ARC-Seal: i=1; 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dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=efPR6AGm; spf=pass (imf23.hostedemail.com: domain of mtosatti@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=mtosatti@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1660763824-325744 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: This patchset contains enhancements on top of Aaron's -v6 of the series (see the changelog below). It fixes the following problems two problems: 1) A customer provided some evidence which indicates that the idle tick was stopped; albeit, CPU-specific vmstat counters still remained populated. Thus one can only assume quiet_vmstat() was not invoked on return to the idle loop. If I understand correctly, I suspect this divergence might erroneously prevent a reclaim attempt by kswapd. If the number of zone specific free pages are below their per-cpu drift value then zone_page_state_snapshot() is used to compute a more accurate view of the aforementioned statistic. Thus any task blocked on the NUMA node specific pfmemalloc_wait queue will be unable to make significant progress via direct reclaim unless it is killed after being woken up by kswapd (see throttle_direct_reclaim()). 2) With a SCHED_FIFO task that busy loops on a given CPU, and kworker for that CPU at SCHED_OTHER priority, queuing work to sync per-vmstats will either cause that work to never execute, or stalld boosts kworker priority which causes a latency violation. Follows the v6 cover letter, with updated changelog. The numbers, for the test program attached at the end of this cover letter, executed inside a KVM VM, are: Vanilla Patch cycles per idle loop 151858 153258 (+1.0%) cycles per syscall 8461 8690 (+2.6%) -------- I have incorporated an idea from Marcelo's patch [1] where a CPU-specific variable is used to indicate if a vmstat differential/or imbalance is present for a given CPU. So, at the appropriate time, vmstat processing can be initiated. The hope is that this particular approach is "cheaper" when compared to need_update() - used currently; in the context of nohz_full and the scheduling-clock tick being stopped, we would now with this patch, check if a CPU-specific vmstat imbalance is present before exiting user-mode (see tick_nohz_user_enter_prepare()). This trivial test program [2] was used to determine the somewhat impact under vanilla and with the proposed changes; mlock(2) and munlock(2) was used solely to modify vmstat item 'NR_MLOCK'. The following is an average count of CPU-cycles across the aforementioned system calls and the idle loop, respectively. I believe these results are negligible: Modified | Vanilla | cycles per syscall: 7399 | cycles per syscall: 4150 cycles per idle loop: 141048 | cycles per idle loop: 144730 | Any feedback would be appreciated. Thanks. Changes since v6 [6]: - sync vmstats independently of whether vmstat_update work is queued or not - clean vmstat_dirty before differential sync loop - cancel pending work if tick stopped - do not queue work to remote CPU if tick stopped Changes since v5 [3]: - Introduced __tick_nohz_user_enter_prepare() - Switched to EXPORT_SYMBOL_GPL() Changes since v4 [4]: - Moved vmstat_dirty specific changes into a separate patch (Marcelo Tosatti) Changes since v3 [5]: - Used EXPORT_SYMBOL() on tick_nohz_user_enter_prepare() - Replaced need_update() - Introduced CPU-specific variable namely vmstat_dirty and mark_vmstat_dirty() [1]: https://lore.kernel.org/lkml/20220204173554.763888172@fedora.localdomain/ [2]: https://pastebin.com/8AtzSAuK [3]: https://lore.kernel.org/lkml/20220801234258.134609-1-atomlin@redhat.com/ [4]: https://lore.kernel.org/lkml/20220621172207.1501641-1-atomlin@redhat.com/ [5]: https://lore.kernel.org/lkml/20220422193647.3808657-1-atomlin@redhat.com/ [6]: https://lore.kernel.org/linux-mm/20220808194820.676246-1-atomlin@redhat.com/ include/linux/tick.h | 5 +++-- kernel/time/tick-sched.c | 19 ++++++++++++++++++- mm/vmstat.c | 74 ++++++++++++++++++++++++++++++++++++-------------------------------------- 3 files changed, 57 insertions(+), 41 deletions(-) --- test-vmstat-overhead.c --- #include #include #include #include #include typedef unsigned long long cycles_t; typedef unsigned long long usecs_t; typedef unsigned long long u64; #ifdef __x86_64__ #define DECLARE_ARGS(val, low, high) unsigned long low, high #define EAX_EDX_VAL(val, low, high) ((low) | ((u64)(high) << 32)) #define EAX_EDX_ARGS(val, low, high) "a" (low), "d" (high) #define EAX_EDX_RET(val, low, high) "=a" (low), "=d" (high) #else #define DECLARE_ARGS(val, low, high) unsigned long long val #define EAX_EDX_VAL(val, low, high) (val) #define EAX_EDX_ARGS(val, low, high) "A" (val) #define EAX_EDX_RET(val, low, high) "=A" (val) #endif static inline unsigned long long __rdtscll(void) { DECLARE_ARGS(val, low, high); asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high)); return EAX_EDX_VAL(val, low, high); } #define rdtscll(val) do { (val) = __rdtscll(); } while (0) #define NRSYSCALLS 30000 #define NRSLEEPS 100000 void main(int argc, char *argv[]) { unsigned long a, b, cycles; int i, syscall = 0; void *page = malloc(4096); if (mlock(page, 4096)) perror("mlock"); if (munlock(page, 4096)) perror("munlock"); if (argc != 2) { printf("usage: %s {idle,syscall}\n", argv[0]); exit(1); } rdtscll(a); if (strncmp("idle", argv[1], 4) == 0) syscall = 0; else if (strncmp("syscall", argv[1], 7) == 0) syscall = 1; else { printf("usage: %s {idle,syscall}\n", argv[0]); exit(1); } if (syscall == 1) { for (i = 0; i < NRSYSCALLS; i++) { if (mlock(page, 4096)) perror("mlock"); if (munlock(page, 4096)) perror("munlock"); } } else { for (i = 0; i < NRSLEEPS; i++) usleep(10); } rdtscll(b); cycles = b - a; if (syscall == 1) printf("cycles per syscall: %d\n", (b-a)/(NRSYSCALLS*2)); else printf("cycles per idle loop: %d\n", (b-a)/NRSLEEPS); }