From: Michal Hocko <mhocko@suse.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Christoph Lameter <cl@linux.com>,
Aaron Tomlin <atomlin@atomlin.com>,
Frederic Weisbecker <frederic@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Russell King <linux@armlinux.org.uk>,
Huacai Chen <chenhuacai@kernel.org>,
Heiko Carstens <hca@linux.ibm.com>,
x86@kernel.org, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v7 00/13] fold per-CPU vmstats remotely
Date: Mon, 20 Mar 2023 19:25:55 +0100 [thread overview]
Message-ID: <ZBilM1JR2HKElIR1@dhcp22.suse.cz> (raw)
In-Reply-To: <20230320180332.102837832@redhat.com>
On Mon 20-03-23 15:03:32, Marcelo Tosatti wrote:
> This patch series addresses the following two problems:
>
> 1. A customer provided evidence indicating that a process
> was stalled in direct reclaim:
>
This is addressed by the trivial patch 1.
[...]
> 2. With a task that busy loops on a given CPU,
> the kworker interruption to execute vmstat_update
> is undesired and may exceed latency thresholds
> for certain applications.
Yes it can but why does that matter?
> By having vmstat_shepherd flush the per-CPU counters to the
> global counters from remote CPUs.
>
> This is done using cmpxchg to manipulate the counters,
> both CPU locally (via the account functions),
> and remotely (via cpu_vm_stats_fold).
>
> Thanks to Aaron Tomlin for diagnosing issue 1 and writing
> the initial patch series.
>
>
> Performance details for the kworker interruption:
>
> oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
> oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ...
> oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
> kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...
>
> The example above shows an additional 7us for the
>
> oslat -> kworker -> oslat
>
> switches. In the case of a virtualized CPU, and the vmstat_update
> interruption in the host (of a qemu-kvm vcpu), the latency penalty
> observed in the guest is higher than 50us, violating the acceptable
> latency threshold for certain applications.
I do not think we have ever promissed any specific latency guarantees
for vmstat. These are statistics have been mostly used for debugging
purposes AFAIK. I am not aware of any specific user space use case that
would be latency sensitive. Your changelog doesn't go into details there
either.
[...]
> mm/vmstat.c | 440 +++++++++++++++++++++++++++++++++++++++++++++++------------------------------
This requires much more detailed story why we really need that.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2023-03-20 18:35 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-20 18:03 [PATCH v7 00/13] fold per-CPU vmstats remotely Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot Marcelo Tosatti
2023-03-20 18:21 ` Michal Hocko
2023-03-20 18:32 ` Marcelo Tosatti
2023-03-22 10:03 ` Michal Hocko
2023-03-20 18:03 ` [PATCH v7 02/13] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 03/13] this_cpu_cmpxchg: loongarch: " Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 04/13] this_cpu_cmpxchg: S390: " Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 05/13] this_cpu_cmpxchg: x86: " Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 06/13] add this_cpu_cmpxchg_local and asm-generic definitions Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 07/13] convert this_cpu_cmpxchg users to this_cpu_cmpxchg_local Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 08/13] mm/vmstat: switch counter modification to cmpxchg Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 09/13] vmstat: switch per-cpu vmstat counters to 32-bits Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 10/13] mm/vmstat: use xchg in cpu_vm_stats_fold Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 11/13] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 12/13] mm/vmstat: refresh stats remotely instead of via work item Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 13/13] vmstat: add pcp remote node draining via cpu_vm_stats_fold Marcelo Tosatti
2023-03-20 20:43 ` Tim Chen
2023-03-22 1:20 ` Marcelo Tosatti
2023-03-20 18:25 ` Michal Hocko [this message]
2023-03-20 19:07 ` [PATCH v7 00/13] fold per-CPU vmstats remotely Marcelo Tosatti
2023-03-22 10:13 ` Michal Hocko
2023-03-22 11:23 ` Marcelo Tosatti
2023-03-22 13:35 ` Michal Hocko
2023-03-22 14:20 ` Marcelo Tosatti
2023-03-23 7:51 ` Michal Hocko
2023-03-23 10:52 ` Marcelo Tosatti
2023-03-23 10:59 ` Marcelo Tosatti
2023-03-23 12:17 ` Michal Hocko
2023-03-23 13:30 ` Marcelo Tosatti
2023-03-23 13:32 ` Marcelo Tosatti
2023-04-18 22:02 ` Andrew Morton
2023-04-19 11:14 ` Marcelo Tosatti
2023-04-19 11:15 ` Marcelo Tosatti
2023-04-19 13:44 ` Andrew Theurer
2023-04-20 7:55 ` Michal Hocko
2023-04-23 1:25 ` Marcelo Tosatti
2023-04-19 11:29 ` Marcelo Tosatti
2023-04-19 11:59 ` Marcelo Tosatti
2023-04-19 12:24 ` Frederic Weisbecker
2023-04-19 13:48 ` Marcelo Tosatti
2023-04-19 14:35 ` Michal Hocko
2023-04-19 16:35 ` Marcelo Tosatti
2023-04-20 8:40 ` Michal Hocko
2023-04-23 1:10 ` Marcelo Tosatti
2023-04-20 13:45 ` Marcelo Tosatti
2023-04-26 14:34 ` Marcelo Tosatti
2023-04-27 8:31 ` Michal Hocko
2023-04-27 14:59 ` Marcelo Tosatti
2023-04-26 15:04 ` Vlastimil Babka
2023-04-26 16:10 ` Marcelo Tosatti
2023-04-27 8:39 ` Michal Hocko
2023-04-27 16:25 ` Marcelo Tosatti
2023-04-19 16:47 ` Vlastimil Babka
2023-04-19 19:15 ` Marcelo Tosatti
2023-05-03 13:51 ` Marcelo Tosatti
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