linux-kernel - Re: [PATCH] x86/resctrl: fix min_cbm

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Date:   Mon, 16 May 2022 12:07:31 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     linux-kernel@...r.kernel.org, fenghua.yu@...el.com,
        babu.moger@....com, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] x86/resctrl: fix min_cbm_bits for AMD

On Mon, May 16, 2022 at 9:35 AM Reinette Chatre
<reinette.chatre@...el.com> wrote:
>
> + x86 maintainers
>
> Hi Stephane,
>
> Thank you very much for catching this. While the fix is onto something
> I would prefer the fix to be obvious and not a side effect of bit
> checking in an empty bitmap.
>
Are you asking for me to add a comment on the modified line or are you asking
for a change in cbm_validate()? There, I could add an empty_bitmask check to
that if.

> When resubmitting, please ensure the subject starts with an uppercase letter.
>
ok.

> Also, when resubmitting, could you please add x86@...nel.org? The resctrl
> patches are routed upstream by the x86 architecture maintainers.
>
ok.

> On 5/15/2022 10:50 PM, Stephane Eranian wrote:
> > AMD supports cbm with no bits set as reflected in rdt_init_res_defs_amd() by:
>
> Please watch the line lengths. Getting a pass from
> scripts/checkpatch.pl would help.
>
> >
> >       r->cache.arch_has_empty_bitmaps = true;
> >
>
> With the above the needed information is present. Changing min_cbm_bits
> is not required.
>
> > However given the unified code in cbm_validate(), checking for
> >
> >     val == 0 && !arch_has_empty_bitmaps
> >
> > is not enough because you also have in cbm_validate():
> >       if ((zero_bit - first_bit) < r->cache.min_cbm_bits)
>
> You are correct, but that relies on checking of bits in a bitmap
> where no bits are set so this fix relies on the failure cases of the earlier
> find_first_bit() and find_next_zero_bit() to be used. I find that it
> obscures the scenario being handled.
>
> The code should be clear and to that end I think it would be simpler to
> explicitly check that an empty bitmap is provided and not search
> for bits at all when that is the case.
>
> Something like this before the bit parsing starts:
>         if (r->cache.arch_has_empty_bitmaps && val == 0)
>                 goto done;
>
>         /* Skip bit parsing */
>
> done:
>         *data = val;
>         return true;
>
> >
> > Default value for if r->cache.min_cbm_bits = 1
> >
> > Leading to:
> >
> > $ cd /sys/fs/resctrl
> > $ mkdir foo
> > $ cd foo
> > $ echo L3:0=0 > schemata
> > -bash: echo: write error: Invalid argument
> >
> > Patch initializes r->cache.min_cbm_bits to 0 for AMD.
> >
>
> Seems like a Fixes tag is needed.
> Fixes: 316e7f901f5a ("x86/resctrl: Add struct rdt_cache::arch_has_{sparse, empty}_bitmaps")
>
> > Signed-off-by: Stephane Eranian <eranian@...gle.com>
> > ---
> >  arch/x86/kernel/cpu/resctrl/core.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> > index bb1c3f5f60c8..14782361ebb7 100644
> > --- a/arch/x86/kernel/cpu/resctrl/core.c
> > +++ b/arch/x86/kernel/cpu/resctrl/core.c
> > @@ -897,6 +897,7 @@ static __init void rdt_init_res_defs_amd(void)
> >                       r->cache.arch_has_sparse_bitmaps = true;
> >                       r->cache.arch_has_empty_bitmaps = true;
> >                       r->cache.arch_has_per_cpu_cfg = true;
> > +                     r->cache.min_cbm_bits = 0;
> >               } else if (r->rid == RDT_RESOURCE_MBA) {
> >                       hw_res->msr_base = MSR_IA32_MBA_BW_BASE;
> >                       hw_res->msr_update = mba_wrmsr_amd;
>
> Reinette