Re: [PATCH] mm: introduce sysctl file to flush per-cpu vmstat statistics

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Nov 20, 2020 at 06:20:06PM +0000, Christopher Lameter wrote:
> On Tue, 17 Nov 2020, Marcelo Tosatti wrote:
> 
> > > So what we would need would be something like a sysctl that puts the
> > > system into a quiet state by completing all workqueue items. Idle all
> > > subsystems that need it and put the cpu into NOHZ mode.
> >
> > Are you suggesting that instead of a specific file to control vmstat
> > workqueue only, a more generic sysctl could be used?
> 
> Yes. Introduce a sysctl to quiet down the system. Clean caches that will
> trigger kernel threads and whatever else is pending on that processor.

OK, makes sense.

About sysctl, in particular:

NOTES         top

       Use of this system call was long discouraged: since Linux 2.6.24,
       uses of this system call result in warnings in the kernel log, and in
       Linux 5.5, the system call was finally removed.  Use the /proc/sys
       interface instead.

       Note that on older kernels where this system call still exists, it is
       available only if the kernel was configured with the
       CONFIG_SYSCTL_SYSCALL option.  Furthermore, glibc does not provide a
       wrapper for this system call, necessitating the use of syscall(2).

So will stick to the /proc/ suggestion.

> > About NOHZ mode: the CPU should enter NOHZ automatically as soon as
> > there is a single thread running, so unclear why that would be needed.
> 
> There are typically pending actions that still trigger interruptions.
> 
> If you would immediately quiet down the system if there is only one thread
> runnable then you would compromise system performance through frequent
> counter folding and cache cleaning etc.

Makes sense.