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From:   "NeilBrown" <neilb@suse.de>
To:     "Chuck Lever" <cel@kernel.org>
Cc:     linux-nfs@vger.kernel.org, "Chuck Lever" <chuck.lever@oracle.com>,
        lorenzo@kernel.org, jlayton@redhat.com, david@fromorbit.com
Subject: Re: [PATCH v2 9/9] SUNRPC: Convert RQ_BUSY into a per-pool bitmap
In-reply-to: <ZKN9xmRn+EN/TQwY@manet.1015granger.net>
References: <168842897573.139194.15893960758088950748.stgit@manet.1015granger.net>,
 <168842930872.139194.10164846167275218299.stgit@manet.1015granger.net>,
 <168843398253.8939.16982425023664424215@noble.neil.brown.name>,
 <ZKN9xmRn+EN/TQwY@manet.1015granger.net>
Date:   Tue, 04 Jul 2023 12:17:28 +1000
Message-id: <168843704829.8939.9406594114602623376@noble.neil.brown.name>
Precedence: bulk

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@oracle.com>
> > >=20
> > > I've noticed that client-observed server request latency goes up
> > > simply when the nfsd thread count is increased.
> > >=20
> > > 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).
> >=20
> > I think this text could usefully be re-written.  By this point in the
> > series we aren't list walking.
> >=20
> > I'd also be curious to know what latency different you get for just this
> > change.
>=20
> Not much of a latency difference at lower thread counts.
>=20
> 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).
>=20
>=20
> > > 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.
> > >=20
> > > 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.
> > >=20
> > > Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> > > ---
> > >  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(-)
> > >=20
> > > 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;
> > > =20
> > >  	/* statistics on pool operation */
> > > @@ -190,6 +191,8 @@ extern u32 svc_max_payload(const struct svc_rqst *r=
qstp);
> > >  #define RPCSVC_MAXPAGES		((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE \
> > >  				+ 2 + 1)
> > > =20
> > > +#define RPCSVC_MAXPOOLTHREADS	(4096)
> > > +
> > >  /*
> > >   * The context of a single thread, including the request currently bei=
ng
> > >   * 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 */
> >=20
> > Might this be a good opportunity to convert this to an enum ??
> >=20
> > >  	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/sunrp=
c.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)
> > > =20
> > >  #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 in=
t 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 =3D
> > > +			bitmap_zalloc_node(RPCSVC_MAXPOOLTHREADS, GFP_KERNEL,
> > > +					   svc_pool_map_get_node(i));
> > > +		if (!pool->sp_idle_map)
> > > +			return NULL;
> > > =20
> > >  		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);
> > > =20
> > >  		xa_destroy(&pool->sp_thread_xa);
> > > +		bitmap_free(pool->sp_idle_map);
> > > +		pool->sp_idle_map =3D NULL;
> > >  	}
> > >  	kfree(serv->sv_pools);
> > >  	kfree(serv);
> > > @@ -647,7 +655,6 @@ svc_rqst_alloc(struct svc_serv *serv, struct svc_po=
ol *pool, int node)
> > > =20
> > >  	folio_batch_init(&rqstp->rq_fbatch);
> > > =20
> > > -	__set_bit(RQ_BUSY, &rqstp->rq_flags);
> > >  	rqstp->rq_server =3D serv;
> > >  	rqstp->rq_pool =3D pool;
> > > =20
> > > @@ -677,7 +684,7 @@ static struct svc_rqst *
> > >  svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int n=
ode)
> > >  {
> > >  	static const struct xa_limit limit =3D {
> > > -		.max =3D U32_MAX,
> > > +		.max =3D 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;
> > > =20
> > > -	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;
> >=20
> > 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.
> >=20
> > > =20
> > > +		rqstp =3D 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_p=
ool *pool, unsigned int *stat
> > >  	}
> > > =20
> > >  found_pool:
> > > -	rqstp =3D xa_find(&pool->sp_thread_xa, &zero, U32_MAX, XA_PRESENT);
> > > +	rqstp =3D xa_find(&pool->sp_thread_xa, &zero, RPCSVC_MAXPOOLTHREADS,
> > > +			XA_PRESENT);
> > >  	if (rqstp) {
> > >  		__xa_erase(&pool->sp_thread_xa, rqstp->rq_thread_id);
> > >  		task =3D 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;
> > >  }
> > > =20
> > > +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();
> > > +}
> >=20
> > 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
> >=20
> >     clear_bit_unlock(rqstp->rq_thread_id, pool->sp_busy_map);
> >=20
> > and  that svc_pool_thread_mark_busy were
> >=20
> >    test_and_set_bit_lock(rqstp->rq_thread_id, pool->sp_busy_map);
> >=20
> > Then it would be more obvious what was happening.
>=20
> 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.

>=20
> I'll try this change for the next version of the series.
>=20

Thanks.

NeilBrown