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

From: Michael Ellerman <mpe@ellerman.id.au>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Nathan Lynch <nathanl@linux.ibm.com>,
	Gautham R Shenoy <ego@linux.vnet.ibm.com>,
	Michael Neuling <mikey@neuling.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Jordan Niethe <jniethe5@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Oliver O'Halloran <oohall@gmail.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v4 08/10] powerpc/smp: Allocate cpumask only after searching thread group
Date: Fri, 31 Jul 2020 22:14:11 +1000	[thread overview]
Message-ID: <87o8nv51bg.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <20200731094938.GA18776@linux.vnet.ibm.com>

Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:
> * Michael Ellerman <mpe@ellerman.id.au> [2020-07-31 17:52:15]:
>
>> Srikar Dronamraju <srikar@linux.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!! )

That's correct.

> 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.

I remember Rusty adding that code, but I don't know if we ever
considered enabling CPUMASK_OFFSTACK.

Probably we meant to but never got around to doing it.

> 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..

Right.

> 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.

Agreed, just clarify in the change log that it's not freed at the moment
because of CPU_MASK_OFFSTACK=n

> 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. 

I don't think so, cpumask_t is semi-deprecated AIUI.

> The reason being we end up using NR_CPU cpumask for each percpu cpumask
> variable instead of using NR_CPU cpumask_t pointer.

Our current defconfigs have NR_CPUS=2048, which is probably just small
enough to continue using OFFSTACK=n.

But we allow configuring NR_CPUS up to 8192, which surely would need
OFFSTACK=y in order to work.

So I think we need to stick with cpumask_var_t, but we should test with
OFFSTACK=y, and should probably be a bit more careful with checking the
allocations succeed.

And then we should select OFFSTACK=y for NR_CPUS above some threshold.

cheers