linux-kernel - Re: [PATCH v4 08/10] powerpc/smp: Allocate cpumask only after searching thread group

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Message-ID: <20200731094938.GA18776@linux.vnet.ibm.com>
Date:   Fri, 31 Jul 2020 15:19:38 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Anton Blanchard <anton@...abs.org>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Michael Neuling <mikey@...ling.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Jordan Niethe <jniethe5@...il.com>
Subject: Re: [PATCH v4 08/10] powerpc/smp: Allocate cpumask only after
 searching thread group

* Michael Ellerman <mpe@...erman.id.au> [2020-07-31 17:52:15]:

> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> > If allocated earlier and the search fails, then cpumask need to be
> > freed. However cpu_l1_cache_map can be allocated after we search thread
> > group.
> 
> It's not freed anywhere AFAICS?
> 

Yes, its never freed. Infact we are never checking if
zalloc_cpumask_var_node fails. Its not just this cpumask, but historically
all the other existing cpumasks in arch/powerpc/kernel/smp.c are never
freed/checked. I did dig into this a bit and it appears that ..
(Please do correct me if I am wrong!! )

Powerpc using cpumask_var_t for all of the percpu variables. And it dont seem
to enable CONFIG_CPUMASK_OFFSTACK even from the MAXSMP config.

So from include/linux/cpumask.h

typedef struct cpumask cpumask_var_t[1];
and
zalloc_cpumask_var_node ends up being cpumask_clear

So I think we are historically we seem to assume we are always
!CPUMASK_OFFSTACK and hence we dont need to check for return as well as
free..

I would look forward to your comments on how we should handle this going
forward. But I would keep this the same for this patchset.

One of the questions that I have is if we most likely are to be in
!CONFIG_CPUMASK_OFFSTACK, then should be migrate to cpumask_t for percpu
variables. 
 
The reason being we end up using NR_CPU cpumask for each percpu cpumask
variable instead of using NR_CPU cpumask_t pointer.

> And even after this change there's still an error path that doesn't free
> it, isn't there?
> 
> cheers
> 
> > Cc: linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
> > Cc: LKML <linux-kernel@...r.kernel.org>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Cc: Nicholas Piggin <npiggin@...il.com>
> > Cc: Anton Blanchard <anton@...abs.org>
> > Cc: Oliver O'Halloran <oohall@...il.com>
> > Cc: Nathan Lynch <nathanl@...ux.ibm.com>
> > Cc: Michael Neuling <mikey@...ling.org>
> > Cc: Gautham R Shenoy <ego@...ux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Valentin Schneider <valentin.schneider@....com>
> > Cc: Jordan Niethe <jniethe5@...il.com>
> > Reviewed-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> > Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kernel/smp.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 698000c7f76f..dab96a1203ec 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -797,10 +797,6 @@ static int init_cpu_l1_cache_map(int cpu)
> >  	if (err)
> >  		goto out;
> >  
> > -	zalloc_cpumask_var_node(&per_cpu(cpu_l1_cache_map, cpu),
> > -				GFP_KERNEL,
> > -				cpu_to_node(cpu));
> > -
> >  	cpu_group_start = get_cpu_thread_group_start(cpu, &tg);
> >  
> >  	if (unlikely(cpu_group_start == -1)) {
> > @@ -809,6 +805,9 @@ static int init_cpu_l1_cache_map(int cpu)
> >  		goto out;
> >  	}
> >  
> > +	zalloc_cpumask_var_node(&per_cpu(cpu_l1_cache_map, cpu),
> > +				GFP_KERNEL, cpu_to_node(cpu));
> > +
> >  	for (i = first_thread; i < first_thread + threads_per_core; i++) {
> >  		int i_group_start = get_cpu_thread_group_start(i, &tg);
> >  
> > -- 
> > 2.17.1

-- 
Thanks and Regards
Srikar Dronamraju