svn commit: r363930 - head/sys/kern

[Mateusz Guzik]

Author: mjg
Date: Wed Aug  5 19:15:59 2020
New Revision: 363930
URL: https://svnweb.freebsd.org/changeset/base/363930

Log:
  pipe: reduce atime precision
  
  The routine is called on successful write and read, which on pipes happens a
  lot and for small sizes.
  
  Precision provided by default seems way bigger than necessary and it causes
  problems in vms on amd64 (it rdtscp's which vmexits). getnanotime seems to
  provide the level roughly in lines of Linux so we should be good here.
  
  Sample result from will-it-scale pipe1_processes -t 1 (ops/s):
  before: 426464
  after: 3247421
  
  Note the that atime handling for named pipes is broken with and without the
  patch. The filesystem code is never used for updating atime and never looks
  at the updated field. Consequently, while there are no provisions added to
  handle named pipes separately, the change is a nop for that case.
  
  Differential Revision:	 https://reviews.freebsd.org/D23964

Modified:
  head/sys/kern/sys_pipe.c

Modified: head/sys/kern/sys_pipe.c
==============================================================================
--- head/sys/kern/sys_pipe.c	Wed Aug  5 19:11:31 2020	(r363929)
+++ head/sys/kern/sys_pipe.c	Wed Aug  5 19:15:59 2020	(r363930)
@@ -230,6 +230,7 @@ static int pipe_create(struct pipe *pipe, bool backing
 static int pipe_paircreate(struct thread *td, struct pipepair **p_pp);
 static __inline int pipelock(struct pipe *cpipe, int catch);
 static __inline void pipeunlock(struct pipe *cpipe);
+static void pipe_timestamp(struct timespec *tsp);
 #ifndef PIPE_NODIRECT
 static int pipe_build_write_buffer(struct pipe *wpipe, struct uio *uio);
 static void pipe_destroy_write_buffer(struct pipe *wpipe);
@@ -279,7 +280,7 @@ pipe_zone_ctor(void *mem, int size, void *arg, int fla
 	 */
 	rpipe = &pp->pp_rpipe;
 	bzero(rpipe, sizeof(*rpipe));
-	vfs_timestamp(&rpipe->pipe_ctime);
+	pipe_timestamp(&rpipe->pipe_ctime);
 	rpipe->pipe_atime = rpipe->pipe_mtime = rpipe->pipe_ctime;
 
 	wpipe = &pp->pp_wpipe;
@@ -422,6 +423,20 @@ pipe_dtor(struct pipe *dpipe)
 }
 
 /*
+ * Get a timestamp.
+ *
+ * This used to be vfs_timestamp but the higher precision is unnecessary and
+ * can very negatively affect performance in virtualized environments (e.g., on
+ * vms running on amd64 when using the rdtscp instruction).
+ */
+static void
+pipe_timestamp(struct timespec *tsp)
+{
+
+	getnanotime(tsp);
+}
+
+/*
  * The pipe system call for the DTYPE_PIPE type of pipes.  If we fail, let
  * the zone pick up the pieces via pipeclose().
  */
@@ -804,7 +819,7 @@ locked_error:
 
 	/* XXX: should probably do this before getting any locks. */
 	if (error == 0)
-		vfs_timestamp(&rpipe->pipe_atime);
+		pipe_timestamp(&rpipe->pipe_atime);
 unlocked_error:
 	--rpipe->pipe_busy;
 
@@ -1290,7 +1305,7 @@ pipe_write(struct file *fp, struct uio *uio, struct uc
 		error = 0;
 
 	if (error == 0)
-		vfs_timestamp(&wpipe->pipe_mtime);
+		pipe_timestamp(&wpipe->pipe_mtime);
 
 	/*
 	 * We have something to offer,