[PATCH] mm/vmalloc: reduce the number of lazy_max_pages to reduce latency
From: Chris Wilson <hidden>
Date: 2016-09-29 08:19:25
Also in:
linux-mm, lkml
Subsystem:
memory management, the rest, vmalloc · Maintainers:
Andrew Morton, Linus Torvalds, Uladzislau Rezki
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