Re: [PATCH 04/34] sched: add cpumask_find_and_set() and use it in __mm_cid_get()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2023-11-20 06:31, Peter Zijlstra wrote:
> On Sat, Nov 18, 2023 at 07:50:35AM -0800, Yury Norov wrote:
>> __mm_cid_get() uses a __mm_cid_try_get() helper to atomically acquire a
>> bit in mm cid mask. Now that we have atomic find_and_set_bit(), we can
>> easily extend it to cpumasks and use in the scheduler code.
>>
>> __mm_cid_try_get() has an infinite loop, which may delay forward
>> progress of __mm_cid_get() when the mask is dense. The
>> cpumask_find_and_set() doesn't poll the mask infinitely, and returns as
>> soon as nothing has found after the first iteration, allowing to acquire
>> the lock, and set use_cid_lock faster, if needed.
> 
> Methieu, I forgot again, but the comment delete seems to suggest you did
> this on purpose...

See comments below.

> 
>> cpumask_find_and_set() considers cid mask as a volatile region of memory,
>> as it actually is in this case. So, if it's changed while search is in
>> progress, KCSAN wouldn't fire warning on it.
>>
>> Signed-off-by: Yury Norov <yury.norov@gmail.com>
>> ---
>>   include/linux/cpumask.h | 12 ++++++++++
>>   kernel/sched/sched.h    | 52 ++++++++++++-----------------------------
>>   2 files changed, 27 insertions(+), 37 deletions(-)
>>
>> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
>> index cfb545841a2c..c2acced8be4e 100644
>> --- a/include/linux/cpumask.h
>> +++ b/include/linux/cpumask.h
>> @@ -271,6 +271,18 @@ unsigned int cpumask_next_and(int n, const struct cpumask *src1p,
>>   		small_cpumask_bits, n + 1);
>>   }
>>   
>> +/**
>> + * cpumask_find_and_set - find the first unset cpu in a cpumask and
>> + *			  set it atomically
>> + * @srcp: the cpumask pointer
>> + *
>> + * Return: >= nr_cpu_ids if nothing is found.
>> + */
>> +static inline unsigned int cpumask_find_and_set(volatile struct cpumask *srcp)
>> +{
>> +	return find_and_set_bit(cpumask_bits(srcp), small_cpumask_bits);
>> +}
>> +
>>   /**
>>    * for_each_cpu - iterate over every cpu in a mask
>>    * @cpu: the (optionally unsigned) integer iterator
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 2e5a95486a42..b2f095a9fc40 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -3345,28 +3345,6 @@ static inline void mm_cid_put(struct mm_struct *mm)
>>   	__mm_cid_put(mm, mm_cid_clear_lazy_put(cid));
>>   }
>>   
>> -static inline int __mm_cid_try_get(struct mm_struct *mm)
>> -{
>> -	struct cpumask *cpumask;
>> -	int cid;
>> -
>> -	cpumask = mm_cidmask(mm);
>> -	/*
>> -	 * Retry finding first zero bit if the mask is temporarily
>> -	 * filled. This only happens during concurrent remote-clear
>> -	 * which owns a cid without holding a rq lock.
>> -	 */
>> -	for (;;) {
>> -		cid = cpumask_first_zero(cpumask);
>> -		if (cid < nr_cpu_ids)
>> -			break;
>> -		cpu_relax();
>> -	}
>> -	if (cpumask_test_and_set_cpu(cid, cpumask))
>> -		return -1;

This was split in find / test_and_set on purpose because following
patches I have (implementing numa-aware mm_cid) have a scan which
needs to scan sets of two cpumasks in parallel (with "and" and
and_not" operators).

Moreover, the "mask full" scenario only happens while a concurrent
remote-clear temporarily owns a cid without rq lock. See
sched_mm_cid_remote_clear():

         /*
          * The cid is unused, so it can be unset.
          * Disable interrupts to keep the window of cid ownership without rq
          * lock small.
          */
         local_irq_save(flags);
         if (try_cmpxchg(&pcpu_cid->cid, &lazy_cid, MM_CID_UNSET))
                 __mm_cid_put(mm, cid);
         local_irq_restore(flags);

The proposed patch here turns this scenario into something heavier
(setting the use_cid_lock) rather than just retrying. I guess the
question to ask here is whether it is theoretically possible to cause
__mm_cid_try_get() to fail to have forward progress if we have a high
rate of sched_mm_cid_remote_clear. If we decide that this is indeed
a possible progress-failure scenario, then it makes sense to fallback
to use_cid_lock as soon as a full mask is encountered.

However, removing the __mm_cid_try_get() helper will make it harder to
integrate the following numa-awareness patches I have on top.

I am not against using cpumask_find_and_set, but can we keep the
__mm_cid_try_get() helper to facilitate integration of future work ?
We just have to make it use cpumask_find_and_set, which should be
easy.

>> -	return cid;
>> -}
>> -
>>   /*
>>    * Save a snapshot of the current runqueue time of this cpu
>>    * with the per-cpu cid value, allowing to estimate how recently it was used.
>> @@ -3381,25 +3359,25 @@ static inline void mm_cid_snapshot_time(struct rq *rq, struct mm_struct *mm)
>>   
>>   static inline int __mm_cid_get(struct rq *rq, struct mm_struct *mm)
>>   {
>> +	struct cpumask *cpumask = mm_cidmask(mm);
>>   	int cid;
>>   
>> -	/*
>> -	 * All allocations (even those using the cid_lock) are lock-free. If
>> -	 * use_cid_lock is set, hold the cid_lock to perform cid allocation to
>> -	 * guarantee forward progress.
>> -	 */
>> +	/* All allocations (even those using the cid_lock) are lock-free. */
>>   	if (!READ_ONCE(use_cid_lock)) {
>> -		cid = __mm_cid_try_get(mm);
>> -		if (cid >= 0)
>> +		cid = cpumask_find_and_set(cpumask);
>> +		if (cid < nr_cpu_ids)
>>   			goto end;
>> -		raw_spin_lock(&cid_lock);
>> -	} else {
>> -		raw_spin_lock(&cid_lock);
>> -		cid = __mm_cid_try_get(mm);
>> -		if (cid >= 0)
>> -			goto unlock;
>>   	}
>>   
>> +	/*
>> +	 * If use_cid_lock is set, hold the cid_lock to perform cid
>> +	 * allocation to guarantee forward progress.
>> +	 */
>> +	raw_spin_lock(&cid_lock);
>> +	cid = cpumask_find_and_set(cpumask);
>> +	if (cid < nr_cpu_ids)
>> +		goto unlock;

In the !use_cid_lock case where we already failed a lookup above, this change
ends up doing another attempt at lookup before setting the use_cid_lock and
attempting again until success. I am not sure what is the motivation for changing
the code flow here ?

General comment about the rest of the series: please review code comments for
typos.

Thanks,

Mathieu

>> +
>>   	/*
>>   	 * cid concurrently allocated. Retry while forcing following
>>   	 * allocations to use the cid_lock to ensure forward progress.
>> @@ -3415,9 +3393,9 @@ static inline int __mm_cid_get(struct rq *rq, struct mm_struct *mm)
>>   	 * all newcoming allocations observe the use_cid_lock flag set.
>>   	 */
>>   	do {
>> -		cid = __mm_cid_try_get(mm);
>> +		cid = cpumask_find_and_set(cpumask);
>>   		cpu_relax();
>> -	} while (cid < 0);
>> +	} while (cid >= nr_cpu_ids);
>>   	/*
>>   	 * Allocate before clearing use_cid_lock. Only care about
>>   	 * program order because this is for forward progress.
>> -- 
>> 2.39.2
>>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com