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From: Frederic Weisbecker <frederic@kernel.org>
To: Hillf Danton <hdanton@sina.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	atomlin@atomlin.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v13 3/6] mm/vmstat: manage per-CPU stats from CPU context when NOHZ full
Date: Tue, 10 Jan 2023 17:12:22 +0100	[thread overview]
Message-ID: <Y72OZn6mULPX4xk8@lothringen> (raw)
In-Reply-To: <20230110151901.402-1-hdanton@sina.com>

On Tue, Jan 10, 2023 at 11:19:01PM +0800, Hillf Danton wrote:
> On Tue, 10 Jan 2023 08:50:28 -0300 Marcelo Tosatti <mtosatti@redhat.com>
> > On Tue, Jan 10, 2023 at 10:43:56AM +0800, Hillf Danton wrote:
> > > On 9 Jan 2023 11:12:49 -0300 Marcelo Tosatti <mtosatti@redhat.com>
> > > > 
> > > > Yes, but if you do not return to userspace, then the per-CPU vm
> > > > statistics can be dirty indefinitely.
> > > 
> > > Could you specify the reasons for failing to return to userspace,
> > > given it is undesired intereference for the shepherd to queue work
> > > on the isolated CPUs.
> > 
> > Any system call that takes longer than the threshold to sync vmstats.
> 
> Which ones?
> 
> If schedule() occurs during syscall because of acquiring mutex for instance
> then anything on the isolated runqueue, including workqueue worker shepherd
> wakes up, can burn CPU cycles without undesired intereference produced.

The above confuses me. How others tasks would help with syscalls that take too long too
service?

> > 
> > Or a long running kernel thread, for example:
> 
> It is a buggyyyy example.
> > 
> > https://stackoverflow.com/questions/65111483/long-running-kthread-and-synchronize-net

I can imagine a CPU spending most of its time processing networking packets
through interrupts/softirq within ksoftirqd/NAPI while another CPU process
these packets in userspace.

In this case the CPU handling the kernel part can theoretically never go to
idle/user. nohz_full isn't optimized toward such job but there is nothing
to prevent it from doing such job.

Thanks.


  parent reply	other threads:[~2023-01-10 16:12 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-05 12:52 [PATCH v13 0/6] Ensure quiet_vmstat() is called when returning to userpace and when idle tick is stopped Marcelo Tosatti
2023-01-05 12:52 ` [PATCH v13 1/6] mm/vmstat: Add CPU-specific variable to track a vmstat discrepancy Marcelo Tosatti
2023-01-10 11:58   ` Christoph Lameter
2023-01-10 12:12     ` Frederic Weisbecker
2023-01-05 12:52 ` [PATCH v13 2/6] mm/vmstat: Use vmstat_dirty to track CPU-specific vmstat discrepancies Marcelo Tosatti
2023-01-10 12:06   ` Christoph Lameter
2023-01-10 12:18     ` Frederic Weisbecker
2023-01-10 13:39       ` Christoph Lameter
2023-01-10 20:09         ` Marcelo Tosatti
2023-01-11  8:42           ` Christoph Lameter
2023-01-11 17:07             ` Marcelo Tosatti
2023-01-16  9:51               ` Christoph Lameter
2023-01-16 16:11                 ` Marcelo Tosatti
2023-01-17 12:52                   ` Christoph Lameter
2023-01-05 12:52 ` [PATCH v13 3/6] mm/vmstat: manage per-CPU stats from CPU context when NOHZ full Marcelo Tosatti
2023-01-05 12:52 ` [PATCH v13 4/6] tick/nohz_full: Ensure quiet_vmstat() is called on exit to user-mode when the idle tick is stopped Marcelo Tosatti
2023-01-05 12:52 ` [PATCH v13 5/6] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too Marcelo Tosatti
2023-01-05 12:52 ` [PATCH v13 6/6] mm/vmstat: avoid queueing work item if cpu stats are clean Marcelo Tosatti
     [not found] ` <20230106001244.4463-1-hdanton@sina.com>
2023-01-06 12:51   ` [PATCH v13 3/6] mm/vmstat: manage per-CPU stats from CPU context when NOHZ full Marcelo Tosatti
     [not found]   ` <20230106150154.4560-1-hdanton@sina.com>
2023-01-06 18:16     ` Marcelo Tosatti
     [not found]     ` <20230107001529.4617-1-hdanton@sina.com>
2023-01-09 14:12       ` Marcelo Tosatti
     [not found]       ` <20230110024356.336-1-hdanton@sina.com>
2023-01-10 11:50         ` Marcelo Tosatti
     [not found]         ` <20230110151901.402-1-hdanton@sina.com>
2023-01-10 16:12           ` Frederic Weisbecker [this message]
     [not found]           ` <20230110235822.456-1-hdanton@sina.com>
2023-01-11  0:09             ` Frederic Weisbecker

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