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

[Chris Wilson]

On Thu, Sep 29, 2016 at 03:34:11PM +0800, Jisheng Zhang wrote:
> On Marvell berlin arm64 platforms, I see the preemptoff tracer report
> a max 26543 us latency at __purge_vmap_area_lazy, this latency is an
> awfully bad for STB. And the ftrace log also shows __free_vmap_area
> contributes most latency now. I noticed that Joel mentioned the same
> issue[1] on x86 platform and gave two solutions, but it seems no patch
> is sent out for this purpose.
> 
> This patch adopts Joel's first solution, but I use 16MB per core
> rather than 8MB per core for the number of lazy_max_pages. After this
> patch, the preemptoff tracer reports a max 6455us latency, reduced to
> 1/4 of original result.

My understanding is that

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 91f44e78c516..3f7c6d6969ac 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -626,7 +626,6 @@ 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);
        struct llist_node *valist;
        struct vmap_area *va;
        struct vmap_area *n_va;
@@ -637,12 +636,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
         * should not expect such behaviour. This just simplifies locking for
         * the case that isn't actually used at the moment anyway.
         */
-       if (!sync && !force_flush) {
-               if (!spin_trylock(&purge_lock))
-                       return;
-       } else
-               spin_lock(&purge_lock);
-
        if (sync)
                purge_fragmented_blocks_allcpus();
 
@@ -667,7 +660,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
                        __free_vmap_area(va);
                spin_unlock(&vmap_area_lock);
        }
-       spin_unlock(&purge_lock);
 }
 
 /*


should now be safe. That should significantly reduce the preempt-disabled
section, I think.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre