linux-kernel - Re: [PATCH v3] PM: EM: Fix potential division-by-zero error in em_compute

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Message-ID: <CAJZ5v0iq4bw3WAk1yQRP=B3zk-rRYwibKNRjSfu=PGqTt6RNYw@mail.gmail.com>
Date: Wed, 16 Apr 2025 13:58:55 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Yaxiong Tian <iambestgod@...com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, lukasz.luba@....com, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Yaxiong Tian <tianyaxiong@...inos.cn>
Subject: Re: [PATCH v3] PM: EM: Fix potential division-by-zero error in em_compute_costs()

On Wed, Apr 16, 2025 at 4:57 AM Yaxiong Tian <iambestgod@...com> wrote:
>
> 在 2025/4/16 01:17, Rafael J. Wysocki 写道:
> > On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@...com> wrote:
> >>
> >> From: Yaxiong Tian <tianyaxiong@...inos.cn>
> >>
> >> When the device is of a non-CPU type, table[i].performance won't be
> >> initialized in the previous em_init_performance(), resulting in division
> >> by zero when calculating costs in em_compute_costs().
> >>
> >> Since the 'cost' algorithm is only used for EAS energy efficiency
> >> calculations and is currently not utilized by other device drivers, we
> >> should add the _is_cpu_device(dev) check to prevent this division-by-zero
> >> issue.
> >>
> >> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
> >
> > Please look at the Fixes: tags in the kernel git history.  They don't
> > look like the one above.
> >
> Yes, there's an extra '<>' here.
>
> >> Signed-off-by: Yaxiong Tian <tianyaxiong@...inos.cn>
> >> ---
> >>   kernel/power/energy_model.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> >> index d9b7e2b38c7a..fc972cc1fc12 100644
> >> --- a/kernel/power/energy_model.c
> >> +++ b/kernel/power/energy_model.c
> >> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
> >>
> >>          /* Compute the cost of each performance state. */
> >>          for (i = nr_states - 1; i >= 0; i--) {
> >> -               unsigned long power_res, cost;
> >> +               unsigned long power_res, cost = 0;
> >>
> >>                  if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
> >>                          ret = cb->get_cost(dev, table[i].frequency, &cost);
> >> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
> >>                                          cost, ret);
> >>                                  return -EINVAL;
> >>                          }
> >> -               } else {
> >> +               } else if (_is_cpu_device(dev)) {
> >
> > Can't you just check this upfront at the beginning of the function and
> > make it bail out if dev is not a CPU device?
> >
> Sure, But the current implementation applies em_compute_costs() to both
> non-CPU devices and CPU devices.

Maybe it shouldn't do that for non-CPU ones?

> After carefully reviewing the latest code,
> I've found this issue has expanded in scope.
>
> There are currently three call paths for invoking em_compute_costs():
>
> 1) Registering performance domains (for both non-CPU and CPU devices)
> em_dev_register_perf_domain() → em_create_pd() →
> em_create_perf_table() → em_compute_costs()
>
> 2)EM update paths (CPU devices only)
>
> Periodic 1000ms update check via em_update_work work item:
> em_check_capacity_update() → em_adjust_new_capacity() →
> em_recalc_and_update() → em_compute_costs()
>
> Exynos-chip initialization:
> em_dev_update_chip_binning() → em_recalc_and_update() → em_compute_costs()
>
> 3) Device cost computation (non-CPU devices only - currently unused)
> em_dev_compute_costs() → em_compute_costs()

So because this one is unused and AFAICS the cost values are never
used for non-CPU devices, it's better to just avoid computing them at
all.

> Note: In em_dev_compute_costs(), when calling em_compute_costs(),
> neither the callback (cb) nor flags are set.In fact, it either does
> nothing at all or performs incorrect operations.
>
> Therefore, should we mandate that non-CPU devices must provide a
> get_cost callback?

Why would that be an improvement?

> So Should we add a check at the beginning of the em_compute_costs() to:
>
>         if (!_is_cpu_device(dev) && !cb->get_cost) {
>                 dev_dbg(dev, "EM: No get_cost provided, cost unset.\n");
>                 return 0;
>         }
> And Modify em_dev_compute_costs() to require callers to provide the cb
> callback function,Also need to update its corresponding comments.
>
>
> >>                          /* increase resolution of 'cost' precision */
> >>                          power_res = table[i].power * 10;
> >>                          cost = power_res / table[i].performance;
> >> --

I think until there is a user of em_dev_compute_costs() this is all
moot and hard to figure out.

I would drop em_dev_compute_costs() altogether for now and put a
_is_cpu_device(dev) upfront check into em_compute_costs().