On Sun, Nov 13, 2011 at 6:17 PM, Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx> wrote:
> flush_all() is called for each kmem_cahce_destroy(). So every cache
> being destroyed dynamically ended up sending an IPI to each CPU in the
> system, regardless if the cache has ever been used there.
>
> For example, if you close the Infinband ipath driver char device file,
> the close file ops calls kmem_cache_destroy(). So running some
> infiniband config tool on one a single CPU dedicated to system tasks
> might interrupt the rest of the 127 CPUs I dedicated to some CPU
> intensive task.
>
> I suspect there is a good chance that every line in the output of "git
> grep kmem_cache_destroy linux/ | grep '\->'" has a similar scenario.
>
> This patch attempts to rectify this issue by sending an IPI to flush
> the per cpu objects back to the free lists only to CPUs that seems to
> have such objects.
>
> The check which CPU to IPI is racy but we don't care since asking a
> CPU without per cpu objects to flush does no damage and as far as I
> can tell the flush_all by itself is racy against allocs on remote
> CPUs anyway, so if you meant the flush_all to be determinstic, you
> had to arrange for locking regardless.
>
> Without this patch the following artificial test case:
>
> $ cd /sys/kernel/slab
> $ for DIR in *; do cat $DIR/alloc_calls > /dev/null; done
>
> produces 166 IPIs on an cpuset isolated CPU. With it it produces none.
>
> The code path of memory allocation failure for CPUMASK_OFFSTACK=y
> config was tested using fault injection framework.
>
> Signed-off-by: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
> Acked-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: Russell King <linux@xxxxxxxxxxxxxxxx>
> CC: linux-mm@xxxxxxxxx
> CC: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> CC: Pekka Enberg <penberg@xxxxxxxxxx>
> CC: Matt Mackall <mpm@xxxxxxxxxxx>
> CC: Sasha Levin <levinsasha928@xxxxxxxxx>
> CC: Rik van Riel <riel@xxxxxxxxxx>
> CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
> ---
> mm/slub.c | 15 ++++++++++++++-
> 1 files changed, 14 insertions(+), 1 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 7d2a996..caf4b3a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2006,7 +2006,20 @@ static void flush_cpu_slab(void *d)
>
> static void flush_all(struct kmem_cache *s)
> {
> - on_each_cpu(flush_cpu_slab, s, 1);
> + cpumask_var_t cpus;
> + struct kmem_cache_cpu *c;
> + int cpu;
> +
> + if (likely(zalloc_cpumask_var(&cpus, GFP_ATOMIC))) {
Perhaps, the technique of local_cpu_mask defined in kernel/sched_rt.c
could be used to replace the above atomic allocation.
Best regards
Hillf
> + for_each_online_cpu(cpu) {
> + c = per_cpu_ptr(s->cpu_slab, cpu);
> + if (c && c->page)
> + cpumask_set_cpu(cpu, cpus);
> + }
> + on_each_cpu_mask(cpus, flush_cpu_slab, s, 1);
> + free_cpumask_var(cpus);
> + } else
> + on_each_cpu(flush_cpu_slab, s, 1);
> }
>
> /*
> --
> 1.7.0.4
>
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