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Thu, 9 Feb 2023 12:32:51 -0300 (-03) Message-ID: <20230209153204.901518530@redhat.com> User-Agent: quilt/0.67 Date: Thu, 09 Feb 2023 12:02:00 -0300 From: Marcelo Tosatti To: Christoph Lameter Cc: Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Marcelo Tosatti Subject: [PATCH v2 10/11] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely References: <20230209150150.380060673@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 X-Rspamd-Queue-Id: B6BD4180024 X-Stat-Signature: 35upib1cpt6ghbcbq7e3pek9td6bubc6 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1675956800-810007 X-HE-Meta: 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 9xDz31vh ZPpuIBITdd9yRbtlyoZniCz0p+1abdUm4Jb5hiKkwl707/MDWeaOhMfA4E9o4XMQfbYBIMvg9iBxK2iVAYzSupnKXwfLoQ3CkwUez70Sridh1FevDZnqr2dxUg8vW4yAmnKltpb2J6xfJdpbAk3P6hHPiyQbRQh3e+TTVqUAmTYdGMKNml6LQo2yvWbX+Ojndf82oXRQ02RQaqfk= 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: Now that the counters are modified via cmpxchg both CPU locally (via the account functions), and remotely (via cpu_vm_stats_fold), its possible to switch vmstat_shepherd to perform the per-CPU vmstats folding remotely. This fixes the following 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 (i.e. stall daemon) boosts kworker priority which causes a latency violation Signed-off-by: Marcelo Tosatti Index: linux-2.6/mm/vmstat.c =================================================================== --- linux-2.6.orig/mm/vmstat.c +++ linux-2.6/mm/vmstat.c @@ -2007,6 +2007,23 @@ static void vmstat_shepherd(struct work_ static DECLARE_DEFERRABLE_WORK(shepherd, vmstat_shepherd); +#ifdef CONFIG_HAVE_CMPXCHG_LOCAL +/* Flush counters remotely if CPU uses cmpxchg to update its per-CPU counters */ +static void vmstat_shepherd(struct work_struct *w) +{ + int cpu; + + cpus_read_lock(); + for_each_online_cpu(cpu) { + cpu_vm_stats_fold(cpu); + cond_resched(); + } + cpus_read_unlock(); + + schedule_delayed_work(&shepherd, + round_jiffies_relative(sysctl_stat_interval)); +} +#else static void vmstat_shepherd(struct work_struct *w) { int cpu; @@ -2026,6 +2043,7 @@ static void vmstat_shepherd(struct work_ schedule_delayed_work(&shepherd, round_jiffies_relative(sysctl_stat_interval)); } +#endif static void __init start_shepherd_timer(void) {