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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Nov 30, 2020 at 09:31:47AM +0000, Christoph Lameter wrote:
> On Fri, 27 Nov 2020, Marcelo Tosatti wrote:
> 
> > Decided to switch to prctl interface, and then it starts
> > to become similar to "task mode isolation" patchset API.
> 
> Right I think that was a good approach.
> 
> > In addition to quiescing pending activities on the CPU, it would
> > also be useful to assign a per-task attribute (which is then assigned
> > to a per-CPU attribute), indicating whether that CPU is running
> > an isolated task or not.
> 
> Sounds good but what would this do? Give a warning like the isolation
> patchset?

Could be. Or disallow certain operations such as CPU hotplug, ring buffer
resize, etc.

> > This per-CPU attribute can be used to, for example, return -EBUSY
> > from ring_buffer_resize() (or any other IPI generating activity
> > which can return an error to userspace).
> 
> Yes good.
> 
> > So rather than:
> >
> > 	prctl(PR_QUIESCE_CPU)	(current interface, similar to
> > 	initial message on the thread but with prctl rather than
> > 	sysfs)
> >
> > To be called before real time loop, one would have:
> >
> > 	prctl(PR_SET_TASK_ISOLATION, ISOLATION_ENABLE) [1]
> > 	real time loop
> > 	prctl(PR_SET_TASK_ISOLATION, ISOLATION_DISABLE)
> >
> > (with the attribute also being cleared on task exit).
> >
> > The general description would be:
> >
> > "Set task isolated mode for a given task, returning an error
> > if the task is not pinned to a single CPU.
> >
> > In this mode, the kernel will avoid interruptions to isolated
> > CPUs when possible."
> >
> > Any objections against such an interface ?
> 
> Maybe do both like in the isolation patchset?
> 
> Often code can tolerate a few interruptions (in some code branches
> regular syscalls may be needed) but one wants the thread to be
> as quiet as possible.

Yes, thanks.