[4/5] cifs: prevent starvation in wait_for_free_credits for multi-credit requests
diff mbox series

Message ID 20190228062405.32107-5-lsahlber@redhat.com
State New
Headers show
Series
  • Simplify credits handling for compound requests
Related show

Commit Message

Ronnie Sahlberg Feb. 28, 2019, 6:24 a.m. UTC
Reserve the last MAX_COMPOUND credits for any request asking for >1 credit.
This is to prevent future compound requests from becoming starved while waiting
for potentially many requests is there is a large number of concurrent
singe-credit requests.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
---
 fs/cifs/transport.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

Comments

Pavel Shilovsky March 2, 2019, 1:41 a.m. UTC | #1
ср, 27 февр. 2019 г. в 22:26, Ronnie Sahlberg <lsahlber@redhat.com>:
>
> Reserve the last MAX_COMPOUND credits for any request asking for >1 credit.
> This is to prevent future compound requests from becoming starved while waiting
> for potentially many requests is there is a large number of concurrent
> singe-credit requests.
>
> Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
> ---
>  fs/cifs/transport.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
> index baf15194aa3d..4ff832fce2e9 100644
> --- a/fs/cifs/transport.c
> +++ b/fs/cifs/transport.c
> @@ -520,6 +520,29 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
>                         }
>
>                         /*
> +                        * For normal commands, reserve the last MAX_COMPOUND
> +                        * credits to compound requests.
> +                        * Otherwise these compounds could be permanently
> +                        * starved for credits by single-credit requests.
> +                        *
> +                        * To prevent spinning CPU, block this thread until
> +                        * there are >MAX_COMPOUND credits available.
> +                        */
> +                       if (!optype && num_credits == 1 &&

What if server decides to grants us credits gradually? E.g. granted
only 2 during negotiate protocol and session setup. The tree connect
will wait forever here.

> +                           *credits <= MAX_COMPOUND) {
> +                               spin_unlock(&server->req_lock);
> +                               cifs_num_waiters_inc(server);
> +                               rc = wait_event_killable(server->request_q,
> +                                       has_credits(server, credits,
> +                                                   MAX_COMPOUND + 1));
> +                               cifs_num_waiters_dec(server);
> +                               if (rc)
> +                                       return rc;
> +                               spin_lock(&server->req_lock);
> +                               continue;
> +                       }
> +
> +                       /*
>                          * Can not count locking commands against total
>                          * as they are allowed to block on server.
>                          */
> --
> 2.13.6
>


--
Best regards,
Pavel Shilovsky
ronnie sahlberg March 6, 2019, 12:07 a.m. UTC | #2
On Sat, Mar 2, 2019 at 11:41 AM Pavel Shilovsky <piastryyy@gmail.com> wrote:
>
> ср, 27 февр. 2019 г. в 22:26, Ronnie Sahlberg <lsahlber@redhat.com>:
> >
> > Reserve the last MAX_COMPOUND credits for any request asking for >1 credit.
> > This is to prevent future compound requests from becoming starved while waiting
> > for potentially many requests is there is a large number of concurrent
> > singe-credit requests.
> >
> > Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
> > ---
> >  fs/cifs/transport.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
> > index baf15194aa3d..4ff832fce2e9 100644
> > --- a/fs/cifs/transport.c
> > +++ b/fs/cifs/transport.c
> > @@ -520,6 +520,29 @@ wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
> >                         }
> >
> >                         /*
> > +                        * For normal commands, reserve the last MAX_COMPOUND
> > +                        * credits to compound requests.
> > +                        * Otherwise these compounds could be permanently
> > +                        * starved for credits by single-credit requests.
> > +                        *
> > +                        * To prevent spinning CPU, block this thread until
> > +                        * there are >MAX_COMPOUND credits available.
> > +                        */
> > +                       if (!optype && num_credits == 1 &&
>
> What if server decides to grants us credits gradually? E.g. granted
> only 2 during negotiate protocol and session setup. The tree connect
> will wait forever here.

Thanks. You are right.
That should be avoidable by checking for and only doing this if we
have <arbitrary large> number of credits already in flight.

I will send an updated series.

>
> > +                           *credits <= MAX_COMPOUND) {
> > +                               spin_unlock(&server->req_lock);
> > +                               cifs_num_waiters_inc(server);
> > +                               rc = wait_event_killable(server->request_q,
> > +                                       has_credits(server, credits,
> > +                                                   MAX_COMPOUND + 1));
> > +                               cifs_num_waiters_dec(server);
> > +                               if (rc)
> > +                                       return rc;
> > +                               spin_lock(&server->req_lock);
> > +                               continue;
> > +                       }
> > +
> > +                       /*
> >                          * Can not count locking commands against total
> >                          * as they are allowed to block on server.
> >                          */
> > --
> > 2.13.6
> >
>
>
> --
> Best regards,
> Pavel Shilovsky

Patch
diff mbox series

diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index baf15194aa3d..4ff832fce2e9 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -520,6 +520,29 @@  wait_for_free_credits(struct TCP_Server_Info *server, const int num_credits,
 			}
 
 			/*
+			 * For normal commands, reserve the last MAX_COMPOUND
+			 * credits to compound requests.
+			 * Otherwise these compounds could be permanently
+			 * starved for credits by single-credit requests.
+			 *
+			 * To prevent spinning CPU, block this thread until
+			 * there are >MAX_COMPOUND credits available.
+			 */
+			if (!optype && num_credits == 1 &&
+			    *credits <= MAX_COMPOUND) {
+				spin_unlock(&server->req_lock);
+				cifs_num_waiters_inc(server);
+				rc = wait_event_killable(server->request_q,
+					has_credits(server, credits,
+						    MAX_COMPOUND + 1));
+				cifs_num_waiters_dec(server);
+				if (rc)
+					return rc;
+				spin_lock(&server->req_lock);
+				continue;
+			}
+
+			/*
 			 * Can not count locking commands against total
 			 * as they are allowed to block on server.
 			 */