Re: [PATCH v1 05/31] x86/resctrl: Remove rdtgroup from update_cpu_closid_rmid()

Dave Martin
2024-04-12T16:12:18Z

On Mon, Apr 08, 2024 at 08:16:08PM -0700, Reinette Chatre wrote:
> Hi James,
>
> On 3/21/2024 9:50 AM, James Morse wrote:
> > update_cpu_closid_rmid() takes a struct rdtgroup as an argument, which
> > it uses to update the local CPUs default pqr values. This is a problem
> > once the resctrl parts move out to /fs/, as the arch code cannot
> > poke around inside struct rdtgroup.
> >
> > Rename update_cpu_closid_rmid() as resctrl_arch_sync_cpus_defaults()
> > to be used as the target of an IPI, and pass the effective CLOSID
> > and RMID in a new struct.
> >
> > Signed-off-by: James Morse <james.morse@arm.com>
> > ---
> > arch/x86/kernel/cpu/resctrl/rdtgroup.c | 19 +++++++++++++++----
> > include/linux/resctrl.h | 11 +++++++++++
> > 2 files changed, 26 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > index 5d2c1ce5b6b1..18f097fce51e 100644
> > --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> > @@ -341,13 +341,13 @@ static int rdtgroup_cpus_show(struct kernfs_open_file *of,
> > * from update_closid_rmid() is protected against __switch_to() because
> > * preemption is disabled.
> > */
> > -static void update_cpu_closid_rmid(void *info)
> > +void resctrl_arch_sync_cpu_defaults(void *info)
> > {
> > - struct rdtgroup *r = info;
> > + struct resctrl_cpu_sync *r = info;
> >
> > if (r) {
> > this_cpu_write(pqr_state.default_closid, r->closid);
> > - this_cpu_write(pqr_state.default_rmid, r->mon.rmid);
> > + this_cpu_write(pqr_state.default_rmid, r->rmid);
> > }
> >
> > /*
> > @@ -362,11 +362,22 @@ static void update_cpu_closid_rmid(void *info)
> > * Update the PGR_ASSOC MSR on all cpus in @cpu_mask,
> > *
> > * Per task closids/rmids must have been set up before calling this function.
> > + * @r may be NULL.
> > */
> > static void
> > update_closid_rmid(const struct cpumask *cpu_mask, struct rdtgroup *r)
> > {
> > - on_each_cpu_mask(cpu_mask, update_cpu_closid_rmid, r, 1);
> > + struct resctrl_cpu_sync defaults;
> > + struct resctrl_cpu_sync *defaults_p = NULL;
>
> Please maintain reverse fir order.

Or, more tersely as follows?

struct resctrl_cpu_sync defaults, *defaults_p = NULL;

"Reverse fir order" seems to be documented as a preference rather than a
rule.

The declarations can be swapped, but defaults_p is in some sense a weak
pointer to defaults, so it feels a bit strange to declare them backwards.

Alternatively, could we rename defaults_p to p? Given the size of this
function I don't think that impacts clarity.

I'll wait for your opinion on this.

> > +
> > + if (r) {
> > + defaults.closid = r->closid;
> > + defaults.rmid = r->mon.rmid;
> > + defaults_p = &defaults;
> > + }
> > +
> > + on_each_cpu_mask(cpu_mask, resctrl_arch_sync_cpu_defaults, defaults_p,
> > + 1);
> > }
> >
> > static int cpus_mon_write(struct rdtgroup *rdtgrp, cpumask_var_t newmask,
> > diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> > index 6e87bc95f5ea..2b79e4159507 100644
> > --- a/include/linux/resctrl.h
> > +++ b/include/linux/resctrl.h
> > @@ -220,6 +220,17 @@ struct resctrl_schema {
> > u32 num_closid;
> > };
> >
> > +struct resctrl_cpu_sync {
> > + u32 closid;
> > + u32 rmid;
> > +};
> > +
> > +/*
> > + * Update and re-load this CPUs defaults. Called via IPI, takes a pointer to
>
> "this CPU's defaults"?

Ack (also in the commit message).

>
> > + * struct resctrl_cpu_sync, or NULL.
> > + */
>
> Updating the CPU's defaults is not the primary goal of this function and because
> of that I do not think this should be the focus with the main goal (updating
> RMID and CLOSID on CPU) ignored. Specifically, this function only updates
> the defaults if *info is set but it _always_ ensures CPU is running with
> appropriate CLOSID/RMID (which may or may not be from a CPU default).
>
> I think resctrl_arch_sync_cpu_closid_rmid() may be more appropriate
> and the comment needs to elaborate what the function does.
>
> > +void resctrl_arch_sync_cpu_defaults(void *info);

That seems reasonable, and follows the original naming and what the
code does:

What about:

/**
* resctrl_arch_sync_cpu_defaults() - Refresh the CPU's CLOSID and RMID.
* Call via IPI.
* @info: If non-NULL, a pointer to a struct resctrl_cpu_sync specifying
* the new CLOSID and RMID for tasks in the default resctrl ctrl
* and mon group when running on this CPU. If NULL, the default
* CLOSID and RMID are not changed.
*
* This is how reassignment of CPUs and/or tasks to different resctrl groups
* is propagated when requested by the resctrl fs core code.
*
* This function should typically record the per-cpu defaults specified by
* @info (if any), and then reconfigure the CPU's hardware CLOSID and RMID
* for subsequent execution based on @current, in the same way as during a
* task switch.
*/

...?

Cheers
---Dave