On Tue, 04 Jul 2023, Chuck Lever wrote:
> On Tue, Jul 04, 2023 at 11:26:22AM +1000, NeilBrown wrote:
> > On Tue, 04 Jul 2023, Chuck Lever wrote:
> > > From: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > >
> > > I've noticed that client-observed server request latency goes up
> > > simply when the nfsd thread count is increased.
> > >
> > > List walking is known to be memory-inefficient. On a busy server
> > > with many threads, enqueuing a transport will walk the "all threads"
> > > list quite frequently. This also pulls in the cache lines for some
> > > hot fields in each svc_rqst (namely, rq_flags).
> >
> > I think this text could usefully be re-written. By this point in the
> > series we aren't list walking.
> >
> > I'd also be curious to know what latency different you get for just this
> > change.
>
> Not much of a latency difference at lower thread counts.
>
> The difference I notice is that with the spinlock version of
> pool_wake_idle_thread, there is significant lock contention as
> the thread count increases, and the throughput result of my fio
> test is lower (outside the result variance).
>
>
> > > The svc_xprt_enqueue() call that concerns me most is the one in
> > > svc_rdma_wc_receive(), which is single-threaded per CQ. Slowing
> > > down completion handling limits the total throughput per RDMA
> > > connection.
> > >
> > > So, avoid walking the "all threads" list to find an idle thread to
> > > wake. Instead, set up an idle bitmap and use find_next_bit, which
> > > should work the same way as RQ_BUSY but it will touch only the
> > > cachelines that the bitmap is in. Stick with atomic bit operations
> > > to avoid taking the pool lock.
> > >
> > > Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > > ---
> > > include/linux/sunrpc/svc.h | 6 ++++--
> > > include/trace/events/sunrpc.h | 1 -
> > > net/sunrpc/svc.c | 27 +++++++++++++++++++++------
> > > net/sunrpc/svc_xprt.c | 30 ++++++++++++++++++++++++------
> > > 4 files changed, 49 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> > > index 6f8bfcd44250..27ffcf7371d0 100644
> > > --- a/include/linux/sunrpc/svc.h
> > > +++ b/include/linux/sunrpc/svc.h
> > > @@ -35,6 +35,7 @@ struct svc_pool {
> > > spinlock_t sp_lock; /* protects sp_sockets */
> > > struct list_head sp_sockets; /* pending sockets */
> > > unsigned int sp_nrthreads; /* # of threads in pool */
> > > + unsigned long *sp_idle_map; /* idle threads */
> > > struct xarray sp_thread_xa;
> > >
> > > /* statistics on pool operation */
> > > @@ -190,6 +191,8 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
> > > #define RPCSVC_MAXPAGES ((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE \
> > > + 2 + 1)
> > >
> > > +#define RPCSVC_MAXPOOLTHREADS (4096)
> > > +
> > > /*
> > > * The context of a single thread, including the request currently being
> > > * processed.
> > > @@ -239,8 +242,7 @@ struct svc_rqst {
> > > #define RQ_SPLICE_OK (4) /* turned off in gss privacy
> > > * to prevent encrypting page
> > > * cache pages */
> > > -#define RQ_BUSY (5) /* request is busy */
> > > -#define RQ_DATA (6) /* request has data */
> > > +#define RQ_DATA (5) /* request has data */
> >
> > Might this be a good opportunity to convert this to an enum ??
> >
> > > unsigned long rq_flags; /* flags field */
> > > u32 rq_thread_id; /* xarray index */
> > > ktime_t rq_qtime; /* enqueue time */
> > > diff --git a/include/trace/events/sunrpc.h b/include/trace/events/sunrpc.h
> > > index ea43c6059bdb..c07824a254bf 100644
> > > --- a/include/trace/events/sunrpc.h
> > > +++ b/include/trace/events/sunrpc.h
> > > @@ -1676,7 +1676,6 @@ DEFINE_SVCXDRBUF_EVENT(sendto);
> > > svc_rqst_flag(USEDEFERRAL) \
> > > svc_rqst_flag(DROPME) \
> > > svc_rqst_flag(SPLICE_OK) \
> > > - svc_rqst_flag(BUSY) \
> > > svc_rqst_flag_end(DATA)
> > >
> > > #undef svc_rqst_flag
> > > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > > index ef350f0d8925..d0278e5190ba 100644
> > > --- a/net/sunrpc/svc.c
> > > +++ b/net/sunrpc/svc.c
> > > @@ -509,6 +509,12 @@ __svc_create(struct svc_program *prog, unsigned int bufsize, int npools,
> > > INIT_LIST_HEAD(&pool->sp_sockets);
> > > spin_lock_init(&pool->sp_lock);
> > > xa_init_flags(&pool->sp_thread_xa, XA_FLAGS_ALLOC);
> > > + /* All threads initially marked "busy" */
> > > + pool->sp_idle_map =
> > > + bitmap_zalloc_node(RPCSVC_MAXPOOLTHREADS, GFP_KERNEL,
> > > + svc_pool_map_get_node(i));
> > > + if (!pool->sp_idle_map)
> > > + return NULL;
> > >
> > > percpu_counter_init(&pool->sp_messages_arrived, 0, GFP_KERNEL);
> > > percpu_counter_init(&pool->sp_sockets_queued, 0, GFP_KERNEL);
> > > @@ -596,6 +602,8 @@ svc_destroy(struct kref *ref)
> > > percpu_counter_destroy(&pool->sp_threads_starved);
> > >
> > > xa_destroy(&pool->sp_thread_xa);
> > > + bitmap_free(pool->sp_idle_map);
> > > + pool->sp_idle_map = NULL;
> > > }
> > > kfree(serv->sv_pools);
> > > kfree(serv);
> > > @@ -647,7 +655,6 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_pool *pool, int node)
> > >
> > > folio_batch_init(&rqstp->rq_fbatch);
> > >
> > > - __set_bit(RQ_BUSY, &rqstp->rq_flags);
> > > rqstp->rq_server = serv;
> > > rqstp->rq_pool = pool;
> > >
> > > @@ -677,7 +684,7 @@ static struct svc_rqst *
> > > svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
> > > {
> > > static const struct xa_limit limit = {
> > > - .max = U32_MAX,
> > > + .max = RPCSVC_MAXPOOLTHREADS,
> > > };
> > > struct svc_rqst *rqstp;
> > > int ret;
> > > @@ -722,12 +729,19 @@ struct svc_rqst *svc_pool_wake_idle_thread(struct svc_serv *serv,
> > > struct svc_pool *pool)
> > > {
> > > struct svc_rqst *rqstp;
> > > - unsigned long index;
> > > + unsigned long bit;
> > >
> > > - xa_for_each(&pool->sp_thread_xa, index, rqstp) {
> > > - if (test_and_set_bit(RQ_BUSY, &rqstp->rq_flags))
> > > + /* Check the pool's idle bitmap locklessly so that multiple
> > > + * idle searches can proceed concurrently.
> > > + */
> > > + for_each_set_bit(bit, pool->sp_idle_map, pool->sp_nrthreads) {
> > > + if (!test_and_clear_bit(bit, pool->sp_idle_map))
> > > continue;
> >
> > I would really rather the map was "sp_busy_map". (initialised with bitmap_fill())
> > Then you could "test_and_set_bit_lock()" and later "clear_bit_unlock()"
> > and so get all the required memory barriers.
> > What we are doing here is locking a particular thread for a task, so
> > "lock" is an appropriate description of what is happening.
> > See also svc_pool_thread_mark_* below.
> >
> > >
> > > + rqstp = xa_load(&pool->sp_thread_xa, bit);
> > > + if (!rqstp)
> > > + break;
> > > +
> > > WRITE_ONCE(rqstp->rq_qtime, ktime_get());
> > > wake_up_process(rqstp->rq_task);
> > > percpu_counter_inc(&pool->sp_threads_woken);
> > > @@ -767,7 +781,8 @@ svc_pool_victim(struct svc_serv *serv, struct svc_pool *pool, unsigned int *stat
> > > }
> > >
> > > found_pool:
> > > - rqstp = xa_find(&pool->sp_thread_xa, &zero, U32_MAX, XA_PRESENT);
> > > + rqstp = xa_find(&pool->sp_thread_xa, &zero, RPCSVC_MAXPOOLTHREADS,
> > > + XA_PRESENT);
> > > if (rqstp) {
> > > __xa_erase(&pool->sp_thread_xa, rqstp->rq_thread_id);
> > > task = rqstp->rq_task;
> > > diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> > > index 7709120b45c1..2844b32c16ea 100644
> > > --- a/net/sunrpc/svc_xprt.c
> > > +++ b/net/sunrpc/svc_xprt.c
> > > @@ -735,6 +735,25 @@ rqst_should_sleep(struct svc_rqst *rqstp)
> > > return true;
> > > }
> > >
> > > +static void svc_pool_thread_mark_idle(struct svc_pool *pool,
> > > + struct svc_rqst *rqstp)
> > > +{
> > > + smp_mb__before_atomic();
> > > + set_bit(rqstp->rq_thread_id, pool->sp_idle_map);
> > > + smp_mb__after_atomic();
> > > +}
> >
> > There memory barriers above and below bother me. There is no comment
> > telling me what they are protecting against.
> > I would rather svc_pool_thread_mark_idle - which unlocks the thread -
> > were
> >
> > clear_bit_unlock(rqstp->rq_thread_id, pool->sp_busy_map);
> >
> > and that svc_pool_thread_mark_busy were
> >
> > test_and_set_bit_lock(rqstp->rq_thread_id, pool->sp_busy_map);
> >
> > Then it would be more obvious what was happening.
>
> Not obvious to me, but that's very likely because I'm not clear what
> clear_bit_unlock() does. :-)
In general, any "lock" operation (mutex, spin, whatever) is (and must
be) and "acquire" type operations which imposes a memory barrier so that
read requests *after* the lock cannot be satisfied with data from
*before* the lock. The read must access data after the lock.
Conversely any "unlock" operations is a "release" type operation which
imposes a memory barrier so that any write request *before* the unlock
must not be delayed until *after* the unlock. The write must complete
before the unlock.
This is exactly what you would expect of locking - it creates a closed
code region that is properly ordered w.r.t comparable closed regions.
test_and_set_bit_lock() and clear_bit_unlock() provide these expected
semantics for bit operations.
New code should (almost?) never have explicit memory barriers like
smp_mb__after_atomic().
It should use one of the many APIs with _acquire or _release suffixes,
or with the more explicit _lock or _unlock.
>
> I'll try this change for the next version of the series.
>
Thanks.
NeilBrown
