Re: [PATCH 1/4] memcg: use mod_node_page_state to update stats

[Dev Jain]

On 11/02/26 2:54 pm, Shakeel Butt wrote:
> On Wed, Feb 11, 2026 at 05:53:38PM +0900, Harry Yoo wrote:
>> On Wed, Feb 11, 2026 at 01:07:40PM +0530, Dev Jain wrote:
>>> On 10/02/26 9:59 pm, Shakeel Butt wrote:
>>>> On Tue, Feb 10, 2026 at 01:08:49PM +0530, Dev Jain wrote:
>>>> [...]
>>>>>> Oh so it is arm64 specific issue. I tested on x86-64 machine and it solves
>>>>>> the little regression it had before. So, on arm64 all this_cpu_ops i.e. without
>>>>>> double underscore, uses LL/SC instructions. 
>>>>>>
>>>>>> Need more thought on this. 
>>>>>>
>>>>>>>> Also can you confirm whether my analysis of the regression was correct?
>>>>>>>>  Because if it was, then this diff looks wrong - AFAIU preempt_disable()
>>>>>>>>  won't stop an irq handler from interrupting the execution, so this
>>>>>>>>  will introduce a bug for code paths running in irq context.
>>>>>>>>
>>>>>>> I was worried about the correctness too, but this_cpu_add() is safe
>>>>>>> against IRQs and so the stat will be _eventually_ consistent?
>>>>>>>
>>>>>>> Ofc it's so confusing! Maybe I'm the one confused.
>>>>>> Yeah there is no issue with proposed patch as it is making the function
>>>>>> re-entrant safe.
>>>>> Ah yes, this_cpu_add() does the addition in one shot without read-modify-write.
>>>>>
>>>>> I am still puzzled whether the original patch was a bug fix or an optimization.
>>>> The original patch was a cleanup patch. The memcg stats update functions
>>>> were already irq/nmi safe without disabling irqs and that patch did the
>>>> same for the numa stats. Though it seems like that is causing regression
>>>> for arm64 as this_cpu* ops are expensive on arm64. 
>>>>
>>>>> The patch description says that node stat updation uses irq unsafe interface.
>>>>> Therefore, we had foo() calling __foo() nested with local_irq_save/restore. But
>>>>> there were code paths which directly called __foo() - so, your patch fixes a bug right
>>>> No, those places were already disabling irqs and should be fine.
>>> Please correct me if I am missing something here. Simply putting an
>>> if (!irqs_disabled()) -> dump_stack() in __lruvec_stat_mod_folio, before
>>> calling __mod_node_page_state, reveals:
>>>
>>> [ 6.486375] Call trace:
>>> [ 6.486376] show_stack+0x20/0x38 (C)
>>> [ 6.486379] dump_stack_lvl+0x74/0x90
>>> [ 6.486382] dump_stack+0x18/0x28
>>> [ 6.486383] __lruvec_stat_mod_folio+0x160/0x180
>>> [ 6.486385] folio_add_file_rmap_ptes+0x128/0x480
>>> [ 6.486388] set_pte_range+0xe8/0x320
>>> [ 6.486389] finish_fault+0x260/0x508
>>> [ 6.486390] do_fault+0x2d0/0x598
>>> [ 6.486391] __handle_mm_fault+0x398/0xb60
>>> [ 6.486393] handle_mm_fault+0x15c/0x298
>>> [ 6.486394] __get_user_pages+0x204/0xb88
>>> [ 6.486395] populate_vma_page_range+0xbc/0x1b8
>>> [ 6.486396] __mm_populate+0xcc/0x1e0
>>> [ 6.486397] __arm64_sys_mlockall+0x1d4/0x1f8
>>> [ 6.486398] invoke_syscall+0x50/0x120
>>> [ 6.486399] el0_svc_common.constprop.0+0x48/0xf0
>>> [ 6.486400] do_el0_svc+0x24/0x38
>>> [ 6.486400] el0_svc+0x34/0xf0
>>> [ 6.486402] el0t_64_sync_handler+0xa0/0xe8
>>> [ 6.486404] el0t_64_sync+0x198/0x1a0
>>>
>>> Indeed finish_fault() takes a PTL spin lock without irq disablement.
>> That indeed looks incorrect to me.
>> I was assuming __foo() is always called with IRQs disabled!
> Not necessarily. For stats which never get updated in IRQ context, can
> be updated using __foo() with just premption disabled.
>
>>>> I am working on adding batched stats update functionality in the hope
>>>> that will fix the regression.
>>> Thanks! FYI, I have zeroed in the issue on to preempt_disable(). Dropping this
>>> from _pcpu_protect_return solves the regression.
>> That's interesting, why is the cost of preempt disable/enable so high?
>>
> What made you (Dev) so convinced that preempt_disable is that expensive.

As I wrote above, dropping the preempt disable from _pcp_protect_return solved
the regression. So, it hints at the cost of this - although it seems surprising
that this may be expensive, so need to investigate : )

>
>>> Unlike x86, arm64 does a preempt_disable
>>> when doing this_cpu_*. On a cursory look it seems like this is unnecessary - since we
>>> are doing preempt_enable() immediately after reading the pointer, CPU migration is
>>> possible anyways, so there is nothing to be gained by reading pcpu pointer with
>>> preemption disabled. I am investigating whether we can simply drop this in general.
> [...]
>> ... so, removing preempt disable _in general_ is probably not a good idea.
>>
> Yup, I agree here.
>
>> [1] https://lore.kernel.org/all/20190311164837.GD24275@xxxxxxxxxxxxxxxxxxxxxxxxx
>>
>> -- 
>> Cheers,
>> Harry / Hyeonggon
>>