Re: [PATCH v3 10/12] x86/mm: Move flush_tlb_info back to the stack

[Nadav Amit]

> On 19 Mar 2026, at 10:49, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
> 
> On 2026-03-19 00:28:19 [+0200], Nadav Amit wrote:
>>>> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
>>>> index 5a3cdc439e38d..4a7f40c7f939a 100644
>>>> --- a/arch/x86/include/asm/tlbflush.h
>>>> +++ b/arch/x86/include/asm/tlbflush.h
>>>> @@ -227,7 +227,7 @@ struct flush_tlb_info {
>>>> 	u8 stride_shift;
>>>> 	u8 freed_tables;
>>>> 	u8 trim_cpumask;
>>>> -};
>>>> +} __aligned(SMP_CACHE_BYTES);
>>>> 
>>> 
>>> This would work, but you are likely to encounter the same problem PeterZ hit
>>> when I did something similar: in some configurations SMP_CACHE_BYTES is very
>>> large.
> 
> So if capping to 64 is an option does not break performance where one
> would complain, why not. But you did it initially so…

No, I only aligned to SMP_CACHE_BYTES. Then I encountered the described problem,
hence I propose to cap it.

> 
>> Further thinking about it and looking at the rest of the series: wouldn’t it be
>> simpler to put flush_tlb_info and smp_call_function_many_cond()’s
>> cpumask on thread_struct? It would allow to support CONFIG_CPUMASK_OFFSTACK=y
>> case by preallocating cpumask on thread creation.
>> 
>> I’m not sure whether the memory overhead is prohibitive.
> 
> My Debian config has CONFIG_NR_CPUS=8192 which would add 1KiB if we add
> a plain cpumask_t. The allocation based on cpumask_size() would add just
> 8 bytes/ pointer to the struct which should be fine. We could even stash
> the mask in the pointer for CPUs <= 64 on 64bit.
> On RT it would be desired to have the memory and not to fallback to
> waiting with disabled preemption if the allocation fails.
> 
> The flush_tlb_info are around 40 bytes + alignment. Maybe we could try
> stack first if this gets us to acceptable performance.

I know it 1KB sounds a lot, but considering fpu size and other per-task
structures, it’s already almost 6KB. Just raising the option.