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

[Peter Zijlstra <peterz@infradead.org>]

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.