block: add support for IO CPU affinity

This patch adds support for controlling the IO completion CPU of
either all requests on a queue, or on a per-request basis. We export
a sysfs variable (rq_affinity) which, if set, migrates completions
of requests to the CPU that originally submitted it. A bio helper
(bio_set_completion_cpu()) is also added, so that queuers can ask
for completion on that specific CPU.

In testing, this has been show to cut the system time by as much
as 20-40% on synthetic workloads where CPU affinity is desired.

This requires a little help from the architecture, so it'll only
work as designed for archs that are using the new generic smp
helper infrastructure.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 2c0c09034fd2..13aba20edb2d 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -81,6 +81,8 @@ struct bio {
 
         unsigned int            bi_max_vecs;    /* max bvl_vecs we can hold */
 
+        unsigned int            bi_comp_cpu;    /* completion CPU */
+
         struct bio_vec          *bi_io_vec;     /* the actual vec list */
 
         bio_end_io_t            *bi_end_io;
@@ -105,6 +107,7 @@ struct bio {
 #define BIO_BOUNCED     5       /* bio is a bounce bio */
 #define BIO_USER_MAPPED 6       /* contains user pages */
 #define BIO_EOPNOTSUPP  7       /* not supported */
+#define BIO_CPU_AFFINE  8       /* complete bio on same CPU as submitted */
 #define bio_flagged(bio, flag)  ((bio)->bi_flags & (1 << (flag)))
 
 /*
@@ -343,6 +346,14 @@ extern struct bio_vec *bvec_alloc_bs(gfp_t, int, unsigned long *, struct bio_set
 extern unsigned int bvec_nr_vecs(unsigned short idx);
 
 /*
+ * Allow queuer to specify a completion CPU for this bio
+ */
+static inline void bio_set_completion_cpu(struct bio *bio, unsigned int cpu)
+{
+        bio->bi_comp_cpu = cpu;
+}
+
+/*
  * bio_set is used to allow other portions of the IO system to
  * allocate their own private memory pools for bio and iovec structures.
  * These memory pools in turn all allocate from the bio_slab