linux-kernel - Re: [PATCH v3 10/26] fs/resctrl: Improve handling for events that can be read from any CPU

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Message-ID: <aAaqbUk3gZbCan13@agluck-desk3>
Date: Mon, 21 Apr 2025 13:28:29 -0700
From: "Luck, Tony" <tony.luck@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: Fenghua Yu <fenghuay@...dia.com>,
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>,
	Drew Fustini <dfustini@...libre.com>,
	Dave Martin <Dave.Martin@....com>,
	Anil Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH v3 10/26] fs/resctrl: Improve handling for events that
 can be read from any CPU

On Fri, Apr 18, 2025 at 03:54:02PM -0700, Reinette Chatre wrote:
> > @@ -619,7 +622,8 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
> >  			goto out;
> >  		}
> >  		d = container_of(hdr, struct rdt_mon_domain, hdr);
> > -		mon_event_read(&rr, r, d, rdtgrp, &d->hdr.cpu_mask, evtid, false);
> > +		mask = md->any_cpu ? cpu_online_mask : &d->hdr.cpu_mask;
> > +		mon_event_read(&rr, r, d, rdtgrp, mask, evtid, false);
> 
> I do not think this accomplishes the goal of this patch. Looking at mon_event_read() it calls
> cpumask_any_housekeeping(cpumask, RESCTRL_PICK_ANY_CPU) before any of the smp_*() calls.
> 
> 	cpumask_any_housekeeping()
> 	{
> 		...
> 		if (exclude_cpu == RESCTRL_PICK_ANY_CPU)
> 			cpu = cpumask_any(mask);
> 		...
> 	}
> 
> cpumask_any() is just cpumask_first() so it will pick the first CPU in the
> online mask that may not be the current CPU.
> 
> fwiw ... there are some optimizations planned in this area that I have not yet studied:
> https://lore.kernel.org/lkml/20250407153856.133093-1-yury.norov@gmail.com/

I remember Peter complaining[1] about extra context switches when
cpumask_any_housekeeping() was introduced, but it seems that the
discussion died with no fix applied.

The blocking problem is that ARM may not be able to read a counter
on a tick_nohz CPU because it may need to sleep.

Do we need more options for events:

1) Must be read on a CPU in the right domain	// Legacy
2) Can be read from any CPU			// My addtion
3) Must be read on a "housekeeping" CPU		// James' code in upstream
4) Cannot be read on a tick_nohz CPU		// Could be combined with 1 or 2?

> Reinette

[1] https://lore.kernel.org/all/20241031142553.3963058-2-peternewman@google.com/
>