linux-kernel - Re: [PATCH V4 1/2] cgroup/rstat: Helper functions for locking expose trylock

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Date: Thu, 27 Jun 2024 21:06:26 -0400
From: Waiman Long <longman@...hat.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>, tj@...nel.org,
 cgroups@...r.kernel.org, yosryahmed@...gle.com, shakeel.butt@...ux.dev
Cc: hannes@...xchg.org, lizefan.x@...edance.com, kernel-team@...udflare.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 1/2] cgroup/rstat: Helper functions for locking expose
 trylock

On 6/27/24 18:22, Waiman Long wrote:
>
> On 6/27/24 17:18, Jesper Dangaard Brouer wrote:
>> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>
>> ---
>>   kernel/cgroup/rstat.c |   40 ++++++++++++++++++++++++++++++----------
>>   1 file changed, 30 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
>> index fb8b49437573..2a42be3a9bb3 100644
>> --- a/kernel/cgroup/rstat.c
>> +++ b/kernel/cgroup/rstat.c
>> @@ -279,17 +279,30 @@ __bpf_hook_end();
>>    * value -1 is used when obtaining the main lock else this is the CPU
>>    * number processed last.
>>    */
>> -static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int 
>> cpu_in_loop)
>> +static inline bool __cgroup_rstat_trylock(struct cgroup *cgrp, int 
>> cpu_in_loop)
>> +{
>> +    bool locked;
>> +
>> +    locked = spin_trylock_irq(&cgroup_rstat_lock);
>> +    if (!locked)
>> +        trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, !locked);
>> +
>> +    return locked;
>> +}
>> +
>> +static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int 
>> cpu_in_loop,
>> +                       bool check_contention)
>>       __acquires(&cgroup_rstat_lock)
>>   {
>> -    bool contended;
>> +    bool locked = false;
>>   -    contended = !spin_trylock_irq(&cgroup_rstat_lock);
>> -    if (contended) {
>> -        trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, 
>> contended);
>> +    if (check_contention)
>> +        locked = __cgroup_rstat_trylock(cgrp, cpu_in_loop);
>> +
>> +    if (!locked)
>>           spin_lock_irq(&cgroup_rstat_lock);
>> -    }
>> -    trace_cgroup_rstat_locked(cgrp, cpu_in_loop, contended);
>> +
>> +    trace_cgroup_rstat_locked(cgrp, cpu_in_loop, !locked);
>>   }
>>     static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int 
>> cpu_in_loop)
>> @@ -328,7 +341,7 @@ static void cgroup_rstat_flush_locked(struct 
>> cgroup *cgrp)
>>               __cgroup_rstat_unlock(cgrp, cpu);
>>               if (!cond_resched())
>>                   cpu_relax();
>> -            __cgroup_rstat_lock(cgrp, cpu);
>> +            __cgroup_rstat_lock(cgrp, cpu, true);
>>           }
>>       }
>>   }
>> @@ -348,9 +361,16 @@ static void cgroup_rstat_flush_locked(struct 
>> cgroup *cgrp)
>>    */
>>   __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
>>   {
>> +    bool locked;
>> +
>>       might_sleep();
>>   -    __cgroup_rstat_lock(cgrp, -1);
>> +    locked = __cgroup_rstat_trylock(cgrp, -1);
>> +    if (!locked) {
>> +        /* Opportunity to ongoing flush detection */
>> +        __cgroup_rstat_lock(cgrp, -1, false);
>> +    }
>> +
>>       cgroup_rstat_flush_locked(cgrp);
>>       __cgroup_rstat_unlock(cgrp, -1);
>>   }
>> @@ -368,7 +388,7 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp)
>>       __acquires(&cgroup_rstat_lock)
>>   {
>>       might_sleep();
>> -    __cgroup_rstat_lock(cgrp, -1);
>> +    __cgroup_rstat_lock(cgrp, -1, true);
>>       cgroup_rstat_flush_locked(cgrp);
>>   }
>>
>>
> Will it be cleaner to add a "bool *flushed" output parameter to 
> __cgroup_rstat_lock() so that the caller can respond differently 
> whether the flushed flag is set or not? In that way, you don't need to 
> expose a separate trylock() API. Also your commit log is empty.

Looking at the use case in patch 2, I would suggest the following APIs.

- bool cgroup_rstat_lock(struct cgroup *cgrp)
- bool cgroup_rstat_lock_or_flushed(struct cgroup *cgrp)

Both will return a bool indicating a flush has already been done if 
true. The 2nd function will not take the lock if a flush is ongoing. 
Both will wait if a flush is ongoing.

Cheers,
Longman