Re: [patch] tlb flush_data: replace per_cpu with an array

Ravikiran G Thirumalai
2009-01-12T22:51:13Z

Hi Frederik,

On Mon, Jan 12, 2009 at 10:35:42PM +0100, Frederik Deweerdt wrote:
>
>Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xprog.eu>
>
>diff --git a/arch/x86/kernel/tlb_64.c b/arch/x86/kernel/tlb_64.c
>index f8be6f1..c177a1f 100644
>--- a/arch/x86/kernel/tlb_64.c
>+++ b/arch/x86/kernel/tlb_64.c
>@@ -33,7 +33,7 @@
> * To avoid global state use 8 different call vectors.
> * Each CPU uses a specific vector to trigger flushes on other
> * CPUs. Depending on the received vector the target CPUs look into
>- * the right per cpu variable for the flush data.
>+ * the right array slot for the flush data.
> *
> * With more than 8 CPUs they are hashed to the 8 available
> * vectors. The limited global vector space forces us to this right now.
>@@ -54,7 +54,7 @@ union smp_flush_state {
> /* State is put into the per CPU data section, but padded
> to a full cache line because other CPUs can access it and we don't
> want false sharing in the per cpu data segment. */
>-static DEFINE_PER_CPU(union smp_flush_state, flush_state);
>+static union smp_flush_state flush_state[NUM_INVALIDATE_TLB_VECTORS];

Since flush_state has a spinlock, an array of flush_states would end up
having multiples spinlocks on the same L1 cacheline. However, I see that
the union smp_flush_state is already padded to SMP_CACHE_BYTES.

With per-cpu areas, locks belonging to different array elements did
not end up on the same internode cacheline due to per-node allocation of
per-cpu areas, but with a simple array, two locks could end up on the same
internode cacheline.

Hence, can you please change the padding in smp_flush_state to
INTERNODE_CACHE_BYTES (you have to derive this off INTERNODE_CACHE_SHIFT)
and align it using ____cacheline_internodealigned_in_smp?