Re: [PATCH v2 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling
On Wed, Jul 22, 2020 at 12:27:47PM +0530, Srikar Dronamraju wrote:
> * Gautham R Shenoy [2020-07-22 11:51:14]:
>
> > Hi Srikar,
> >
> > > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > > index 72f16dc0cb26..57468877499a 100644
> > > --- a/arch/powerpc/kernel/smp.c
> > > +++ b/arch/powerpc/kernel/smp.c
> > > @@ -1196,6 +1196,7 @@ static bool update_mask_by_l2(int cpu, struct
> > > cpumask *(*mask_fn)(int))
> > > if (!l2_cache)
> > > return false;
> > >
> > > + cpumask_set_cpu(cpu, mask_fn(cpu));
> >
> >
> > Ok, we need to do this because "cpu" is not yet set in the
> > cpu_online_mask. Prior to your patch the "cpu" was getting set in
> > cpu_l2_cache_map(cpu) as a side-effect of the code that is removed in
> > the patch.
> >
>
> Right.
>
> >
> > > for_each_cpu(i, cpu_online_mask) {
> > > /*
> > >* when updating the marks the current CPU has not been marked
> > > @@ -1278,29 +1279,30 @@ static void add_cpu_to_masks(int cpu)
> > >* add it to it's own thread sibling mask.
> > >*/
> > > cpumask_set_cpu(cpu, cpu_sibling_mask(cpu));
> > > + cpumask_set_cpu(cpu, cpu_core_mask(cpu));
>
> Note: Above, we are explicitly setting the cpu_core_mask.
You are right. I missed this.
>
> > >
> > > for (i = first_thread; i < first_thread + threads_per_core; i++)
> > > if (cpu_online(i))
> > > set_cpus_related(i, cpu, cpu_sibling_mask);
> > >
> > > add_cpu_to_smallcore_masks(cpu);
> > > - /*
> > > - * Copy the thread sibling mask into the cache sibling mask
> > > - * and mark any CPUs that share an L2 with this CPU.
> > > - */
> > > - for_each_cpu(i, cpu_sibling_mask(cpu))
> > > - set_cpus_related(cpu, i, cpu_l2_cache_mask);
> > > update_mask_by_l2(cpu, cpu_l2_cache_mask);
> > >
> > > - /*
> > > - * Copy the cache sibling mask into core sibling mask and mark
> > > - * any CPUs on the same chip as this CPU.
> > > - */
> > > - for_each_cpu(i, cpu_l2_cache_mask(cpu))
> > > - set_cpus_related(cpu, i, cpu_core_mask);
> > > + if (pkg_id == -1) {
> >
> > I suppose this "if" condition is an optimization, since if pkg_id != -1,
> > we anyway set these CPUs in the cpu_core_mask below.
> >
> > However...
>
> This is not just an optimization.
> The hunk removed would only work if cpu_l2_cache_mask is bigger than
> cpu_sibling_mask. (this was the previous assumption that we want to break)
> If the cpu_sibling_mask is bigger than cpu_l2_cache_mask and pkg_id is -1,
> then setting only cpu_l2_cache_mask in cpu_core_mask will result in a broken
> topology.
>
> >
> > > + struct cpumask *(*mask)(int) = cpu_sibling_mask;
> > > +
> > > + /*
> > > + * Copy the sibling mask into core sibling mask and
> > > + * mark any CPUs on the same chip as this CPU.
> > > + */
> > > + if (shared_caches)
> > > + mask = cpu_l2_cache_mask;
> > > +
> > > + for_each_cpu(i, mask(cpu))
> > > + set_cpus_related(cpu, i, cpu_core_mask);
> > >
> > > - if (pkg_id == -1)
> > > return;
> > > + }
> >
> >
> > ... since "cpu" is not yet set in the cpu_online_mask, do we not miss
> > setting
> > "cpu" in the cpu_core_mask(cpu) in the for-loop below ?
> >
> >
>
> As noted above, we are setting before. So we don't missing the cpu and hence
> have not different from before.
Fair enough.
>
> > --
> > Thanks and Regards
> > gautham.
>
> --
> Thanks and Regards
> Srikar Dronamraju
Re: [PATCH v2 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling
* Gautham R Shenoy [2020-07-22 11:51:14]:
> Hi Srikar,
>
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 72f16dc0cb26..57468877499a 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -1196,6 +1196,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask
> > *(*mask_fn)(int))
> > if (!l2_cache)
> > return false;
> >
> > + cpumask_set_cpu(cpu, mask_fn(cpu));
>
>
> Ok, we need to do this because "cpu" is not yet set in the
> cpu_online_mask. Prior to your patch the "cpu" was getting set in
> cpu_l2_cache_map(cpu) as a side-effect of the code that is removed in
> the patch.
>
Right.
>
> > for_each_cpu(i, cpu_online_mask) {
> > /*
> > * when updating the marks the current CPU has not been marked
> > @@ -1278,29 +1279,30 @@ static void add_cpu_to_masks(int cpu)
> > * add it to it's own thread sibling mask.
> > */
> > cpumask_set_cpu(cpu, cpu_sibling_mask(cpu));
> > + cpumask_set_cpu(cpu, cpu_core_mask(cpu));
Note: Above, we are explicitly setting the cpu_core_mask.
> >
> > for (i = first_thread; i < first_thread + threads_per_core; i++)
> > if (cpu_online(i))
> > set_cpus_related(i, cpu, cpu_sibling_mask);
> >
> > add_cpu_to_smallcore_masks(cpu);
> > - /*
> > -* Copy the thread sibling mask into the cache sibling mask
> > -* and mark any CPUs that share an L2 with this CPU.
> > -*/
> > - for_each_cpu(i, cpu_sibling_mask(cpu))
> > - set_cpus_related(cpu, i, cpu_l2_cache_mask);
> > update_mask_by_l2(cpu, cpu_l2_cache_mask);
> >
> > - /*
> > -* Copy the cache sibling mask into core sibling mask and mark
> > -* any CPUs on the same chip as this CPU.
> > -*/
> > - for_each_cpu(i, cpu_l2_cache_mask(cpu))
> > - set_cpus_related(cpu, i, cpu_core_mask);
> > + if (pkg_id == -1) {
>
> I suppose this "if" condition is an optimization, since if pkg_id != -1,
> we anyway set these CPUs in the cpu_core_mask below.
>
> However...
This is not just an optimization.
The hunk removed would only work if cpu_l2_cache_mask is bigger than
cpu_sibling_mask. (this was the previous assumption that we want to break)
If the cpu_sibling_mask is bigger than cpu_l2_cache_mask and pkg_id is -1,
then setting only cpu_l2_cache_mask in cpu_core_mask will result in a broken
topology.
>
> > + struct cpumask *(*mask)(int) = cpu_sibling_mask;
> > +
> > + /*
> > +* Copy the sibling mask into core sibling mask and
> > +* mark any CPUs on the same chip as this CPU.
> > +*/
> > + if (shared_caches)
> > + mask = cpu_l2_cache_mask;
> > +
> > + for_each_cpu(i, mask(cpu))
> > + set_cpus_related(cpu, i, cpu_core_mask);
> >
> > - if (pkg_id == -1)
> > return;
> > + }
>
>
> ... since "cpu" is not yet set in the cpu_online_mask, do we not miss setting
> "cpu" in the cpu_core_mask(cpu) in the for-loop below ?
>
>
As noted above, we are setting before. So we don't missing the cpu and hence
have not different from before.
> --
> Thanks and Regards
> gautham.
--
Thanks and Regards
Srikar Dronamraju
Re: [PATCH v2 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling
Hi Srikar,
On Tue, Jul 21, 2020 at 05:08:09PM +0530, Srikar Dronamraju wrote:
> Current code assumes that cpumask of cpus sharing a l2-cache mask will
> always be a superset of cpu_sibling_mask.
>
> Lets stop that assumption. cpu_l2_cache_mask is a superset of
> cpu_sibling_mask if and only if shared_caches is set.
>
> Cc: linuxppc-dev
> Cc: LKML
> Cc: Michael Ellerman
> Cc: Ingo Molnar
> Cc: Peter Zijlstra
> Cc: Valentin Schneider
> Cc: Nick Piggin
> Cc: Oliver OHalloran
> Cc: Nathan Lynch
> Cc: Michael Neuling
> Cc: Anton Blanchard
> Cc: Gautham R Shenoy
> Cc: Vaidyanathan Srinivasan
> Cc: Jordan Niethe
> Signed-off-by: Srikar Dronamraju
> ---
> Changelog v1 -> v2:
> powerpc/smp: Dont assume l2-cache to be superset of sibling
> Set cpumask after verifying l2-cache. (Gautham)
>
> arch/powerpc/kernel/smp.c | 28 +++-
> 1 file changed, 15 insertions(+), 13 deletions(-)
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 72f16dc0cb26..57468877499a 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1196,6 +1196,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask
> *(*mask_fn)(int))
> if (!l2_cache)
> return false;
>
> + cpumask_set_cpu(cpu, mask_fn(cpu));
Ok, we need to do this because "cpu" is not yet set in the
cpu_online_mask. Prior to your patch the "cpu" was getting set in
cpu_l2_cache_map(cpu) as a side-effect of the code that is removed in
the patch.
> for_each_cpu(i, cpu_online_mask) {
> /*
>* when updating the marks the current CPU has not been marked
> @@ -1278,29 +1279,30 @@ static void add_cpu_to_masks(int cpu)
>* add it to it's own thread sibling mask.
>*/
> cpumask_set_cpu(cpu, cpu_sibling_mask(cpu));
> + cpumask_set_cpu(cpu, cpu_core_mask(cpu));
>
> for (i = first_thread; i < first_thread + threads_per_core; i++)
> if (cpu_online(i))
> set_cpus_related(i, cpu, cpu_sibling_mask);
>
> add_cpu_to_smallcore_masks(cpu);
> - /*
> - * Copy the thread sibling mask into the cache sibling mask
> - * and mark any CPUs that share an L2 with this CPU.
> - */
> - for_each_cpu(i, cpu_sibling_mask(cpu))
> - set_cpus_related(cpu, i, cpu_l2_cache_mask);
> update_mask_by_l2(cpu, cpu_l2_cache_mask);
>
> - /*
> - * Copy the cache sibling mask into core sibling mask and mark
> - * any CPUs on the same chip as this CPU.
> - */
> - for_each_cpu(i, cpu_l2_cache_mask(cpu))
> - set_cpus_related(cpu, i, cpu_core_mask);
> + if (pkg_id == -1) {
I suppose this "if" condition is an optimization, since if pkg_id != -1,
we anyway set these CPUs in the cpu_core_mask below.
However...
> + struct cpumask *(*mask)(int) = cpu_sibling_mask;
> +
> + /*
> + * Copy the sibling mask into core sibling mask and
> + * mark any CPUs on the same chip as this CPU.
> + */
> + if (shared_caches)
> + mask = cpu_l2_cache_mask;
> +
> + for_each_cpu(i, mask(cpu))
> + set_cpus_related(cpu, i, cpu_core_mask);
>
> - if (pkg_id == -1)
> return;
> + }
... since "cpu" is not yet set in the cpu_online_mask, do we not miss setting
"cpu" in the cpu_core_mask(cpu) in the for-loop below ?
>
> for_each_cpu(i, cpu_online_mask)
> if (get_physical_package_id(i) == pkg_id)
Before this patch it was unconditionally getting set in
cpu_core_mask(cpu) because of the fact that it was set in
cpu_l2_cache_mask(cpu) and we were unconditionally setting all the
CPUs in cpu_l2_cache_mask(cpu) in cpu_core_mask(cpu).
What am I missing ?
> --
> 2.17.1
>
--
Thanks and Regards
gautham.
