linux-kernel - Re: [PATCH v2] memcg: Remove the lockdep assert from __mod_objcg

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Date: Tue, 28 May 2024 17:07:06 +0200
From: "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] memcg: Remove the lockdep assert from
 __mod_objcg_mlstate().

On 5/28/24 4:59 PM, Shakeel Butt wrote:
> On Tue, May 28, 2024 at 04:13:41PM GMT, Sebastian Andrzej Siewior wrote:
>> The assert was introduced in the commit cited below as an insurance that
>> the semantic is the same after the local_irq_save() has been removed and
>> the function has been made static.
>> 
>> The original requirement to disable interrupt was due the modification
>> of per-CPU counters which require interrupts to be disabled because the
>> counter update operation is not atomic and some of the counters are
>> updated from interrupt context.
>> 
>> All callers of __mod_objcg_mlstate() acquire a lock
>> (memcg_stock.stock_lock) which disables interrupts on !PREEMPT_RT and
>> the lockdep assert is satisfied. On PREEMPT_RT the interrupts are not
>> disabled and the assert triggers.
>> 
>> The safety of the counter update is already ensured by
>> VM_WARN_ON_IRQS_ENABLED() which is part of __mod_memcg_lruvec_state() and
>> does not require yet another check.
> 
> One question on VM_WARN_ON_IRQS_ENABLED() in __mod_memcg_lruvec_state().
> On a PREEMPT_RT kernel with CONFIG_DEBUG_VM, will that
> VM_WARN_ON_IRQS_ENABLED() cause a splat or VM_WARN_ON_IRQS_ENABLED is
> special on PREEMPT_RT kernels?

It only does something with CONFIG_DEBUG_VM_IRQSOFF and that's disabled by
dependencies on PREEMPT_RT :)

>> 
>> Remove the lockdep assert from __mod_objcg_mlstate().
>> 
>> Fixes: 91882c1617c15 ("memcg: simple cleanup of stats update functions")
>> Link: https://lore.kernel.org/r/20240528121928.i-Gu7Jvg@linutronix.de
>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>> ---
>> On 2024-05-28 15:44:51 [+0200], Vlastimil Babka (SUSE) wrote:
>> > I think just s/memcg_stats_lock()/__mod_memcg_lruvec_state()/ in your
>> > phrasing, since we are removing the lockdep assert from path that calls
>> > __mod_memcg_lruvec_state() and not memcg_stats_lock()?
>> > Or am I missing something?
>> 
>> Yeah, makes sense.
>> 
>>  mm/memcontrol.c |    2 --
>>  1 file changed, 2 deletions(-)
>> 
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3147,8 +3147,6 @@ static inline void __mod_objcg_mlstate(s
>>  	struct mem_cgroup *memcg;
>>  	struct lruvec *lruvec;
>>  
>> -	lockdep_assert_irqs_disabled();
>> -
>>  	rcu_read_lock();
>>  	memcg = obj_cgroup_memcg(objcg);
>>  	lruvec = mem_cgroup_lruvec(memcg, pgdat);