[PATCH RT 7/8] fs: only send IPI to invalidate LRU BH when needed

[Steven Rostedt <rostedt@goodmis.org>]

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3.2.53-rt76-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Gilad Ben-Yossef <gilad@benyossef.com>

[ Upstream commit 42be35d0390b966253136a285f507f5ad00fd9e8 ]

In several code paths, such as when unmounting a file system (but not
only) we send an IPI to ask each cpu to invalidate its local LRU BHs.

For multi-cores systems that have many cpus that may not have any LRU BH
because they are idle or because they have not performed any file system
accesses since last invalidation (e.g.  CPU crunching on high perfomance
computing nodes that write results to shared memory or only using
filesystems that do not use the bh layer.) This can lead to loss of
performance each time someone switches the KVM (the virtual keyboard and
screen type, not the hypervisor) if it has a USB storage stuck in.

This patch attempts to only send an IPI to cpus that have LRU BH.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 fs/buffer.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 997b199..36d49f1 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1425,10 +1425,23 @@ static void invalidate_bh_lru(void *arg)
 	}
 	put_cpu_var(bh_lrus);
 }
+
+static bool has_bh_in_lru(int cpu, void *dummy)
+{
+	struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
+	int i;
 	
+	for (i = 0; i < BH_LRU_SIZE; i++) {
+		if (b->bhs[i])
+			return 1;
+	}
+
+	return 0;
+}
+
 void invalidate_bh_lrus(void)
 {
-	on_each_cpu(invalidate_bh_lru, NULL, 1);
+	on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1, GFP_KERNEL);
 }
 EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
 
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1.8.4.3