libxc: maintain a small, per-handle, cache of hypercall buffer memory (Was: Re: [Xen-devel] Xen 4.1 rc1 test report)

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Wed, 2011-01-26 at 00:47 +0000, Haitao Shan wrote:
> I think it is basically the same idea as Keir introduced in 20841. I
> guess this bug would happen on platforms which has large number of
> physical CPUs, not only on EX system of Intel.
> If you can cook the patch, that would be great! Thanks!!

Does this help?

Ian.

8<---------------------------------------

# HG changeset patch
# User Ian Campbell <ian.campbell@xxxxxxxxxx>
# Date 1296038761 0
# Node ID 8b8b7e024f9d6f4c2ce1a4efbf38f07eeb460d91
# Parent  e4e69622dc95037eab6740f79ecf9c1d05bca529
libxc: maintain a small, per-handle, cache of hypercall buffer memory

Constantly m(un)locking memory can have significant overhead on
systems with large numbers of CPUs. This was previously fixed by
20841:fbe8f32fa257 but this was dropped during the transition to
hypercall buffers.

Introduce a small cache of single page hypercall buffer allocations
which can be reused to avoid this overhead.

Add some statistics tracking to the hypercall buffer allocations.

The cache size of 4 was chosen based on these statistics since they
indicated that 2 pages was sufficient to satisfy all concurrent single
page hypercall buffer allocations seen during "xl create", "xl
shutdown" and "xl destroy" of both a PV and HVM guest therefore 4
pages should cover the majority of important cases.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

diff -r e4e69622dc95 -r 8b8b7e024f9d tools/libxc/xc_hcall_buf.c
--- a/tools/libxc/xc_hcall_buf.c        Wed Jan 26 10:22:42 2011 +0000
+++ b/tools/libxc/xc_hcall_buf.c        Wed Jan 26 10:46:01 2011 +0000
@@ -18,6 +18,7 @@
 
 #include <stdlib.h>
 #include <string.h>
+#include <pthread.h>
 
 #include "xc_private.h"
 #include "xg_private.h"
@@ -28,11 +29,108 @@ xc_hypercall_buffer_t XC__HYPERCALL_BUFF
     HYPERCALL_BUFFER_INIT_NO_BOUNCE
 };
 
+pthread_mutex_t hypercall_buffer_cache_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static void hypercall_buffer_cache_lock(xc_interface *xch)
+{
+    if ( xch->flags & XC_OPENFLAG_NON_REENTRANT )
+        return;
+    pthread_mutex_lock(&hypercall_buffer_cache_mutex);
+}
+
+static void hypercall_buffer_cache_unlock(xc_interface *xch)
+{
+    if ( xch->flags & XC_OPENFLAG_NON_REENTRANT )
+        return;
+    pthread_mutex_unlock(&hypercall_buffer_cache_mutex);
+}
+
+static void *hypercall_buffer_cache_alloc(xc_interface *xch, int nr_pages)
+{
+    void *p = NULL;
+
+    hypercall_buffer_cache_lock(xch);
+
+    xch->hypercall_buffer_total_allocations++;
+    xch->hypercall_buffer_current_allocations++;
+    if ( xch->hypercall_buffer_current_allocations > 
xch->hypercall_buffer_maximum_allocations )
+        xch->hypercall_buffer_maximum_allocations = 
xch->hypercall_buffer_current_allocations;
+
+    if ( nr_pages > 1 )
+    {
+        xch->hypercall_buffer_cache_toobig++;
+    }
+    else if ( xch->hypercall_buffer_cache_nr > 0 )
+    {
+        p = xch->hypercall_buffer_cache[--xch->hypercall_buffer_cache_nr];
+        xch->hypercall_buffer_cache_hits++;
+    }
+    else
+    {
+        xch->hypercall_buffer_cache_misses++;
+    }
+
+    hypercall_buffer_cache_unlock(xch);
+
+    return p;
+}
+
+static int hypercall_buffer_cache_free(xc_interface *xch, void *p, int 
nr_pages)
+{
+    int rc = 0;
+
+    hypercall_buffer_cache_lock(xch);
+
+    xch->hypercall_buffer_total_releases++;
+    xch->hypercall_buffer_current_allocations--;
+
+    if ( nr_pages == 1 && xch->hypercall_buffer_cache_nr < 
HYPERCALL_BUFFER_CACHE_SIZE )
+    {
+        xch->hypercall_buffer_cache[xch->hypercall_buffer_cache_nr++] = p;
+        rc = 1;
+    }
+
+    hypercall_buffer_cache_unlock(xch);
+
+    return rc;
+}
+
+void xc__hypercall_buffer_cache_release(xc_interface *xch)
+{
+    void *p;
+
+    hypercall_buffer_cache_lock(xch);
+
+    DBGPRINTF("hypercall buffer: total allocations:%d total releases:%d",
+              xch->hypercall_buffer_total_allocations,
+              xch->hypercall_buffer_total_releases);
+    DBGPRINTF("hypercall buffer: current allocations:%d maximum 
allocations:%d",
+              xch->hypercall_buffer_current_allocations,
+              xch->hypercall_buffer_maximum_allocations);
+    DBGPRINTF("hypercall buffer: cache current size:%d",
+              xch->hypercall_buffer_cache_nr);
+    DBGPRINTF("hypercall buffer: cache hits:%d misses:%d toobig:%d",
+              xch->hypercall_buffer_cache_hits,
+              xch->hypercall_buffer_cache_misses,
+              xch->hypercall_buffer_cache_toobig);
+
+    while ( xch->hypercall_buffer_cache_nr > 0 )
+    {
+        p = xch->hypercall_buffer_cache[--xch->hypercall_buffer_cache_nr];
+        xch->ops->u.privcmd.free_hypercall_buffer(xch, xch->ops_handle, p, 1);
+    }
+
+    hypercall_buffer_cache_unlock(xch);
+}
+
 void *xc__hypercall_buffer_alloc_pages(xc_interface *xch, 
xc_hypercall_buffer_t *b, int nr_pages)
 {
-    void *p = xch->ops->u.privcmd.alloc_hypercall_buffer(xch, xch->ops_handle, 
nr_pages);
+    void *p = hypercall_buffer_cache_alloc(xch, nr_pages);
 
-    if (!p)
+    if ( !p )
+        p = xch->ops->u.privcmd.alloc_hypercall_buffer(xch, xch->ops_handle, 
nr_pages);
+
+    if ( !p )
         return NULL;
 
     b->hbuf = p;
@@ -47,7 +145,8 @@ void xc__hypercall_buffer_free_pages(xc_
     if ( b->hbuf == NULL )
         return;
 
-    xch->ops->u.privcmd.free_hypercall_buffer(xch, xch->ops_handle, b->hbuf, 
nr_pages);
+    if ( !hypercall_buffer_cache_free(xch, b->hbuf, nr_pages) )
+        xch->ops->u.privcmd.free_hypercall_buffer(xch, xch->ops_handle, 
b->hbuf, nr_pages);
 }
 
 struct allocation_header {
diff -r e4e69622dc95 -r 8b8b7e024f9d tools/libxc/xc_private.c
--- a/tools/libxc/xc_private.c  Wed Jan 26 10:22:42 2011 +0000
+++ b/tools/libxc/xc_private.c  Wed Jan 26 10:46:01 2011 +0000
@@ -126,6 +126,16 @@ static struct xc_interface_core *xc_inte
     xch->error_handler   = logger;           xch->error_handler_tofree   = 0;
     xch->dombuild_logger = dombuild_logger;  xch->dombuild_logger_tofree = 0;
 
+    xch->hypercall_buffer_cache_nr = 0;
+
+    xch->hypercall_buffer_total_allocations = 0;
+    xch->hypercall_buffer_total_releases = 0;
+    xch->hypercall_buffer_current_allocations = 0;
+    xch->hypercall_buffer_maximum_allocations = 0;
+    xch->hypercall_buffer_cache_hits = 0;
+    xch->hypercall_buffer_cache_misses = 0;
+    xch->hypercall_buffer_cache_toobig = 0;
+
     xch->ops_handle = XC_OSDEP_OPEN_ERROR;
     xch->ops = NULL;
 
@@ -171,6 +181,8 @@ static int xc_interface_close_common(xc_
 static int xc_interface_close_common(xc_interface *xch)
 {
     int rc = 0;
+
+    xc_hypercall_buffer_cache_release(xch);
 
     xtl_logger_destroy(xch->dombuild_logger_tofree);
     xtl_logger_destroy(xch->error_handler_tofree);
diff -r e4e69622dc95 -r 8b8b7e024f9d tools/libxc/xc_private.h
--- a/tools/libxc/xc_private.h  Wed Jan 26 10:22:42 2011 +0000
+++ b/tools/libxc/xc_private.h  Wed Jan 26 10:46:01 2011 +0000
@@ -75,6 +75,28 @@ struct xc_interface_core {
     FILE *dombuild_logger_file;
     const char *currently_progress_reporting;
 
+    /*
+     * A simple cache of unused, single page, hypercall buffers
+     *
+     * Protected by a global lock.
+     */
+#define HYPERCALL_BUFFER_CACHE_SIZE 4
+    int hypercall_buffer_cache_nr;
+    void *hypercall_buffer_cache[HYPERCALL_BUFFER_CACHE_SIZE];
+
+    /*
+     * Hypercall buffer statistics. All protected by the global
+     * hypercall_buffer_cache lock.
+     */
+    int hypercall_buffer_total_allocations;
+    int hypercall_buffer_total_releases;
+    int hypercall_buffer_current_allocations;
+    int hypercall_buffer_maximum_allocations;
+    int hypercall_buffer_cache_hits;
+    int hypercall_buffer_cache_misses;
+    int hypercall_buffer_cache_toobig;
+
+    /* Low lovel OS interface */
     xc_osdep_info_t  osdep;
     xc_osdep_ops    *ops; /* backend operations */
     xc_osdep_handle  ops_handle; /* opaque data for xc_osdep_ops */
@@ -156,6 +178,11 @@ int xc__hypercall_bounce_pre(xc_interfac
 #define xc_hypercall_bounce_pre(_xch, _name) xc__hypercall_bounce_pre(_xch, 
HYPERCALL_BUFFER(_name))
 void xc__hypercall_bounce_post(xc_interface *xch, xc_hypercall_buffer_t 
*bounce);
 #define xc_hypercall_bounce_post(_xch, _name) xc__hypercall_bounce_post(_xch, 
HYPERCALL_BUFFER(_name))
+
+/*
+ * Release hypercall buffer cache
+ */
+void xc__hypercall_buffer_cache_release(xc_interface *xch);
 
 /*
  * Hypercall interfaces.



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