[PATCH] mm/vmalloc: reduce the number of lazy_max_pages to reduce latency

[Joel Fernandes]

On Sun, Oct 9, 2016 at 12:26 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Sun, Oct 09, 2016 at 12:00:31PM -0700, Joel Fernandes wrote:
>> Ok. So I'll submit a patch with mutex for purge_lock and use
>> cond_resched_lock for the vmap_area_lock as you suggested. I'll also
>> drop the lazy_max_pages to 8MB as Andi suggested to reduce the lock
>> hold time. Let me know if you have any objections.
>
> The downside of using a mutex here though, is that we may be called
> from contexts that cannot sleep (alloc_vmap_area), or reschedule for
> that matter! If we change the notion of purged, we can forgo the mutex
> in favour of spinning on the direct reclaim path. That just leaves the
> complication of whether to use cond_resched_lock() or a lock around
> the individual __free_vmap_area().

Good point. I agree with you. I think we still need to know if purging
is in progress to preserve previous trylock behavior. How about
something like the following diff? (diff is untested).

This drops the purge lock and uses a ref count to indicate if purging
is in progress, so that callers who don't want to purge if purging is
already in progress can be kept happy. Also I am reducing vmap_lazy_nr
as we go, and, not all at once, so that we don't reduce the counter
too soon as we're not holding purge lock anymore. Lastly, I added the
cond_resched as you suggested.

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f2481cb..5616ca4 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -626,7 +626,7 @@ void set_iounmap_nonlazy(void)
 static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
                                        int sync, int force_flush)
 {
-       static DEFINE_SPINLOCK(purge_lock);
+       static atomic_t purging;
        struct llist_node *valist;
        struct vmap_area *va;
        struct vmap_area *n_va;
@@ -638,10 +638,10 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
         * the case that isn't actually used at the moment anyway.
         */
        if (!sync && !force_flush) {
-               if (!spin_trylock(&purge_lock))
+               if (atomic_cmpxchg(&purging, 0, 1))
                        return;
        } else
-               spin_lock(&purge_lock);
+               atomic_inc(&purging);

        if (sync)
                purge_fragmented_blocks_allcpus();
@@ -655,9 +655,6 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
                nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
        }

-       if (nr)
-               atomic_sub(nr, &vmap_lazy_nr);
-
        if (nr || force_flush)
                flush_tlb_kernel_range(*start, *end);

@@ -665,9 +662,11 @@ static void __purge_vmap_area_lazy(unsigned long
*start, unsigned long *end,
                spin_lock(&vmap_area_lock);
                llist_for_each_entry_safe(va, n_va, valist, purge_list)
                        __free_vmap_area(va);
+               atomic_sub(1, &vmap_lazy_nr);
+               cond_resched_lock(&vmap_area_lock);
                spin_unlock(&vmap_area_lock);
        }
-       spin_unlock(&purge_lock);
+       atomic_dec(&purging);
 }