Re: [PATCH v3] sched/core: Don't use dying mm as active_mm of kthreads

[Waiman Long <longman@redhat.com>]

On 7/30/19 4:43 AM, Peter Zijlstra wrote:
> On Mon, Jul 29, 2019 at 05:07:28PM -0400, Waiman Long wrote:
>> It was found that a dying mm_struct where the owning task has exited
>> can stay on as active_mm of kernel threads as long as no other user
>> tasks run on those CPUs that use it as active_mm. This prolongs the
>> life time of dying mm holding up some resources that cannot be freed
>> on a mostly idle system.
>>
>> Fix that by forcing the kernel threads to use init_mm as the active_mm
>> during a kernel thread to kernel thread transition if the previous
>> active_mm is dying (!mm_users). This will allows the freeing of resources
>> associated with the dying mm ASAP.
>>
>> The presence of a kernel-to-kernel thread transition indicates that
>> the cpu is probably idling with no higher priority user task to run.
>> So the overhead of loading the mm_users cacheline should not really
>> matter in this case.
>>
>> My testing on an x86 system showed that the mm_struct was freed within
>> seconds after the task exited instead of staying alive for minutes or
>> even longer on a mostly idle system before this patch.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  kernel/sched/core.c | 21 +++++++++++++++++++--
>>  1 file changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 795077af4f1a..41997e676251 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3214,6 +3214,8 @@ static __always_inline struct rq *
>>  context_switch(struct rq *rq, struct task_struct *prev,
>>  	       struct task_struct *next, struct rq_flags *rf)
>>  {
>> +	struct mm_struct *next_mm = next->mm;
>> +
>>  	prepare_task_switch(rq, prev, next);
>>  
>>  	/*
>> @@ -3229,8 +3231,22 @@ context_switch(struct rq *rq, struct task_struct *prev,
>>  	 *
>>  	 * kernel ->   user   switch + mmdrop() active
>>  	 *   user ->   user   switch
>> +	 *
>> +	 * kernel -> kernel and !prev->active_mm->mm_users:
>> +	 *   switch to init_mm + mmgrab() + mmdrop()
>>  	 */
>> -	if (!next->mm) {                                // to kernel
>> +	if (!next_mm) {					// to kernel
>> +		/*
>> +		 * Checking is only done on kernel -> kernel transition
>> +		 * to avoid any performance overhead while user tasks
>> +		 * are running.
>> +		 */
>> +		if (unlikely(!prev->mm &&
>> +			     !atomic_read(&prev->active_mm->mm_users))) {
>> +			next_mm = next->active_mm = &init_mm;
>> +			mmgrab(next_mm);
>> +			goto mm_switch;
>> +		}
>>  		enter_lazy_tlb(prev->active_mm, next);
>>  
>>  		next->active_mm = prev->active_mm;
> So I _really_ hate this complication. I'm thinking if you really care
> about this the time is much better spend getting rid of the active_mm
> tracking for x86 entirely.
>
That is fine. I won't pursue further. I will take a look at your
suggestion when I have time, but it will probably be a while :-)

Cheers,
Longman