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SubjectRe: [PATCH v1 2/2] mm: add priority threshold to __purge_vmap_area_lazy()
On Mon, Jan 28, 2019 at 05:45:28PM -0500, Joel Fernandes wrote:
> On Thu, Jan 24, 2019 at 12:56:48PM +0100, Uladzislau Rezki (Sony) wrote:
> > commit 763b218ddfaf ("mm: add preempt points into
> > __purge_vmap_area_lazy()")
> >
> > introduced some preempt points, one of those is making an
> > allocation more prioritized over lazy free of vmap areas.
> >
> > Prioritizing an allocation over freeing does not work well
> > all the time, i.e. it should be rather a compromise.
> >
> > 1) Number of lazy pages directly influence on busy list length
> > thus on operations like: allocation, lookup, unmap, remove, etc.
> >
> > 2) Under heavy stress of vmalloc subsystem i run into a situation
> > when memory usage gets increased hitting out_of_memory -> panic
> > state due to completely blocking of logic that frees vmap areas
> > in the __purge_vmap_area_lazy() function.
> >
> > Establish a threshold passing which the freeing is prioritized
> > back over allocation creating a balance between each other.
>
> I'm a bit concerned that this will introduce the latency back if vmap_lazy_nr
> is greater than half of lazy_max_pages(). Which IIUC will be more likely if
> the number of CPUs is large.
>
The threshold that we establish is two times more than lazy_max_pages(),
i.e. in case of 4 system CPUs lazy_max_pages() is 24576, therefore the
threshold is 49152, if PAGE_SIZE is 4096.

It means that we allow rescheduling if vmap_lazy_nr < 49152. If vmap_lazy_nr
is higher then we forbid rescheduling and free areas until it becomes lower
again to stabilize the system. By doing that, we will not allow vmap_lazy_nr
to be enormously increased.

>
> In fact, when vmap_lazy_nr is high, that's when the latency will be the worst
> so one could say that that's when you *should* reschedule since the frees are
> taking too long and hurting real-time tasks.
>
> Could this be better solved by tweaking lazy_max_pages() such that purging is
> more aggressive?
>
> Another approach could be to detect the scenario you brought up (allocations
> happening faster than free), somehow, and avoid a reschedule?
>
This is what i am trying to achieve by this change.

Thank you for your comments.

--
Vlad Rezki
> >
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> > mm/vmalloc.c | 18 ++++++++++++------
> > 1 file changed, 12 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index fb4fb5fcee74..abe83f885069 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -661,23 +661,27 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> > struct llist_node *valist;
> > struct vmap_area *va;
> > struct vmap_area *n_va;
> > - bool do_free = false;
> > + int resched_threshold;
> >
> > lockdep_assert_held(&vmap_purge_lock);
> >
> > valist = llist_del_all(&vmap_purge_list);
> > + if (unlikely(valist == NULL))
> > + return false;
> > +
> > + /*
> > + * TODO: to calculate a flush range without looping.
> > + * The list can be up to lazy_max_pages() elements.
> > + */
> > llist_for_each_entry(va, valist, purge_list) {
> > if (va->va_start < start)
> > start = va->va_start;
> > if (va->va_end > end)
> > end = va->va_end;
> > - do_free = true;
> > }
> >
> > - if (!do_free)
> > - return false;
> > -
> > flush_tlb_kernel_range(start, end);
> > + resched_threshold = (int) lazy_max_pages() << 1;
> >
> > spin_lock(&vmap_area_lock);
> > llist_for_each_entry_safe(va, n_va, valist, purge_list) {
> > @@ -685,7 +689,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> >
> > __free_vmap_area(va);
> > atomic_sub(nr, &vmap_lazy_nr);
> > - cond_resched_lock(&vmap_area_lock);
> > +
> > + if (atomic_read(&vmap_lazy_nr) < resched_threshold)
> > + cond_resched_lock(&vmap_area_lock);
> > }
> > spin_unlock(&vmap_area_lock);
> > return true;
> > --
> > 2.11.0
> >

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