nptl: Add glibc.pthread.stack_cache_size tunable

The valgrind/helgrind test suite needs a way to make stack dealloction
more prompt, and this feature seems to be generally useful.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

NEWS

@@ -49,6 +49,9 @@ Major new features:
   with names ending with .conf to logically classify the converter modules in
   that directory.
 
+* On Linux, a new tunable, glibc.pthread.stack_cache_size, can be used
+  to configure the size of the thread stack cache.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The function pthread_mutex_consistent_np has been deprecated; programs

manual/tunables.texi

@@ -402,6 +402,15 @@ is acquired.
 The default value of this tunable is @samp{100}.
 @end deftp
 
+@deftp Tunable glibc.pthread.stack_cache_size
+This tunable configures the maximum size of the stack cache.  Once the
+stack cache exceeds this size, unused thread stacks are returned to
+the kernel, to bring the cache size below this limit.
+
+The value is measured in bytes.  The default is @samp{41943040}
+(fourty mibibytes).
+@end deftp
+
 @node Hardware Capability Tunables
 @section Hardware Capability Tunables
 @cindex hardware capability tunables

nptl/nptl-stack.c

@@ -21,8 +21,7 @@
 #include <ldsodefs.h>
 #include <pthreadP.h>
 
-/* Maximum size in kB of cache.  40MiBi by default.  */
-static const size_t stack_cache_maxsize = 40 * 1024 * 1024;
+size_t __nptl_stack_cache_maxsize = 40 * 1024 * 1024;
 
 void
 __nptl_stack_list_del (list_t *elem)
@@ -103,8 +102,9 @@ queue_stack (struct pthread *stack)
   __nptl_stack_list_add (&stack->list, &GL (dl_stack_cache));
 
   GL (dl_stack_cache_actsize) += stack->stackblock_size;
-  if (__glibc_unlikely (GL (dl_stack_cache_actsize) > stack_cache_maxsize))
-    __nptl_free_stacks (stack_cache_maxsize);
+  if (__glibc_unlikely (GL (dl_stack_cache_actsize)
+			> __nptl_stack_cache_maxsize))
+    __nptl_free_stacks (__nptl_stack_cache_maxsize);
 }
 
 void

nptl/nptl-stack.h

@@ -25,6 +25,9 @@
 #include <list.h>
 #include <stdbool.h>
 
+/* Maximum size of the cache, in bytes.  40 MiB by default.  */
+extern size_t __nptl_stack_cache_maxsize attribute_hidden;
+
 /* Check whether the stack is still used or not.  */
 static inline bool
 __nptl_stack_in_use (struct pthread *pd)

nptl/pthread_mutex_conf.c

@@ -23,6 +23,7 @@
 #include <stdbool.h>
 #include <unistd.h>  /* Get STDOUT_FILENO for _dl_printf.  */
 #include <elf/dl-tunables.h>
+#include <nptl-stack.h>
 
 struct mutex_config __mutex_aconf =
 {
@@ -38,10 +39,18 @@ TUNABLE_CALLBACK (set_mutex_spin_count) (tunable_val_t *valp)
   __mutex_aconf.spin_count = (int32_t) (valp)->numval;
 }
 
+static void
+TUNABLE_CALLBACK (set_stack_cache_size) (tunable_val_t *valp)
+{
+  __nptl_stack_cache_maxsize = valp->numval;
+}
+
 void
 __pthread_tunables_init (void)
 {
   TUNABLE_GET (mutex_spin_count, int32_t,
                TUNABLE_CALLBACK (set_mutex_spin_count));
+  TUNABLE_GET (stack_cache_size, size_t,
+               TUNABLE_CALLBACK (set_stack_cache_size));
 }
 #endif

sysdeps/nptl/dl-tunables.list

@@ -23,5 +23,9 @@ glibc {
       maxval: 32767
       default: 100
     }
+    stack_cache_size {
+      type: SIZE_T
+      default: 41943040
+    }
   }
 }