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 <[email protected]> > --- > 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.
