nptl: Invert the mmap/mprotect logic on allocated stacks (BZ#18988)

Current allocate_stack logic for create stacks is to first mmap all
the required memory with the desirable memory and then mprotect the
guard area with PROT_NONE if required.  Although it works as expected,
it pessimizes the allocation because it requires the kernel to actually
increase commit charge (it counts against the available physical/swap
memory available for the system).

The only issue is to actually check this change since side-effects are
really Linux specific and to actually account them it would require a
kernel specific tests to parse the system wide information.  On the kernel
I checked /proc/self/statm does not show any meaningful difference for
vmm and/or rss before and after thread creation.  I could only see
really meaningful information checking on system wide /proc/meminfo
between thread creation: MemFree, MemAvailable, and Committed_AS shows
large difference without the patch.  I think trying to use these
kind of information on a testcase is fragile.

The BZ#18988 reports shows that the commit pages are easily seen with
mlockall (MCL_FUTURE) (with lock all pages that become mapped in the
process) however a more straighfoward testcase shows that pthread_create
could be faster using this patch:

--
static const int inner_count = 256;
static const int outer_count = 128;

static
void *thread1(void *arg)
{
  return NULL;
}

static
void *sleeper(void *arg)
{
  pthread_t ts[inner_count];
  for (int i = 0; i < inner_count; i++)
    pthread_create (&ts[i], &a, thread1, NULL);
  for (int i = 0; i < inner_count; i++)
    pthread_join (ts[i], NULL);

  return NULL;
}

int main(void)
{
  pthread_attr_init(&a);
  pthread_attr_setguardsize(&a, 1<<20);
  pthread_attr_setstacksize(&a, 1134592);

  pthread_t ts[outer_count];
  for (int i = 0; i < outer_count; i++)
    pthread_create(&ts[i], &a, sleeper, NULL);
  for (int i = 0; i < outer_count; i++)
    pthread_join(ts[i], NULL);
    assert(r == 0);
  }
  return 0;
}

--

On x86_64 (4.4.0-45-generic, gcc 5.4.0) running the small benchtests
I see:

$ time ./test

real	0m3.647s
user	0m0.080s
sys	0m11.836s

While with the patch I see:

$ time ./test

real	0m0.696s
user	0m0.040s
sys	0m1.152s

So I added a pthread_create benchtest (thread_create) which check
the thread creation latency.  As for the simple benchtests, I saw
improvements in thread creation on all architectures I tested the
change.

Checked on x86_64-linux-gnu, i686-linux-gnu, aarch64-linux-gnu,
arm-linux-gnueabihf, powerpc64le-linux-gnu, sparc64-linux-gnu,
and sparcv9-linux-gnu.

	[BZ #18988]
	* benchtests/thread_create-inputs: New file.
	* benchtests/thread_create-source.c: Likewise.
	* support/xpthread_attr_setguardsize.c: Likewise.
	* support/Makefile (libsupport-routines): Add
	xpthread_attr_setguardsize object.
	* support/xthread.h: Add xpthread_attr_setguardsize prototype.
	* benchtests/Makefile (bench-pthread): Add thread_create.
	* nptl/allocatestack.c (allocate_stack): Call mmap with PROT_NONE and
	then mprotect the required area.

ChangeLog

@@ -1,3 +1,18 @@
+2017-06-14  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
+
+	[BZ #18988]
+	* benchtests/thread_create-inputs: New file.
+	* benchtests/thread_create-source.c: Likewise.
+	* support/xpthread_attr_setguardsize.c: Likewise.
+	* support/Makefile (libsupport-routines): Add
+	xpthread_attr_setguardsize object.
+	* support/xthread.h: Add xpthread_attr_setguardsize prototype.
+	* benchtests/Makefile (bench-pthread): Add thread_create.
+	* nptl/allocatestack.c (allocate_stack): Call mmap with PROT_NONE and
+	then mprotect the required area.
+	(guard_position): New function.
+	(setup_stack_prot): Likewise.
+
 2017-06-14  H.J. Lu  <hongjiu.lu@intel.com>
 	    Erich Elsen  <eriche@google.com>
 	    Adhemerval Zanella  <adhemerval.zanella@linaro.org>

benchtests/Makefile

@@ -25,7 +25,7 @@ bench-math := acos acosh asin asinh atan atanh cos cosh exp exp2 log log2 \
 	      modf pow rint sin sincos sinh sqrt tan tanh fmin fmax fminf \
 	      fmaxf
 
-bench-pthread := pthread_once
+bench-pthread := pthread_once thread_create
 
 bench-string := ffs ffsll
 

benchtests/thread_create-inputs

@@ -0,0 +1,14 @@
+## args: int:size_t:size_t
+## init: thread_create_init
+## includes: pthread.h
+## include-sources: thread_create-source.c
+
+## name: stack=1024,guard=1
+32, 1024, 1
+## name: stack=1024,guard=2
+32, 1024, 2
+
+## name: stack=2048,guard=1
+32, 2048, 1
+## name: stack=2048,guard=2
+32, 2048, 2

benchtests/thread_create-source.c

@@ -0,0 +1,58 @@
+/* Measure pthread_create thread creation with different stack
+   and guard sizes.
+
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <unistd.h>
+#include <support/xthread.h>
+
+static size_t pgsize;
+
+static void
+thread_create_init (void)
+{
+  pgsize = sysconf (_SC_PAGESIZE);
+}
+
+static void *
+thread_dummy (void *arg)
+{
+  return NULL;
+}
+
+static void
+thread_create (int nthreads, size_t stacksize, size_t guardsize)
+{
+  pthread_attr_t attr;
+  xpthread_attr_init (&attr);
+
+  stacksize = stacksize * pgsize;
+  guardsize = guardsize * pgsize;
+
+  xpthread_attr_setstacksize (&attr, stacksize);
+  xpthread_attr_setguardsize (&attr, guardsize);
+
+  pthread_t ts[nthreads];
+
+  for (int i = 0; i < nthreads; i++)
+    ts[i] = xpthread_create (&attr, thread_dummy, NULL);
+
+  for (int i = 0; i < nthreads; i++)
+    xpthread_join (ts[i]);
+}

nptl/allocatestack.c

@@ -334,6 +334,43 @@ change_stack_perm (struct pthread *pd
   return 0;
 }
 
+/* Return the guard page position on allocated stack.  */
+static inline char *
+__attribute ((always_inline))
+guard_position (void *mem, size_t size, size_t guardsize, struct pthread *pd,
+		size_t pagesize_m1)
+{
+#ifdef NEED_SEPARATE_REGISTER_STACK
+  return mem + (((size - guardsize) / 2) & ~pagesize_m1);
+#elif _STACK_GROWS_DOWN
+  return mem;
+#elif _STACK_GROWS_UP
+  return (char *) (((uintptr_t) pd - guardsize) & ~pagesize_m1);
+#endif
+}
+
+/* Based on stack allocated with PROT_NONE, setup the required portions with
+   'prot' flags based on the guard page position.  */
+static inline int
+setup_stack_prot (char *mem, size_t size, char *guard, size_t guardsize,
+		  const int prot)
+{
+  char *guardend = guard + guardsize;
+#if _STACK_GROWS_DOWN
+  /* As defined at guard_position, for architectures with downward stack
+     the guard page is always at start of the allocated area.  */
+  if (mprotect (guardend, size - guardsize, prot) != 0)
+    return errno;
+#else
+  size_t mprots1 = (uintptr_t) guard - (uintptr_t) mem;
+  if (mprotect (mem, mprots1, prot) != 0)
+    return errno;
+  size_t mprots2 = ((uintptr_t) mem + size) - (uintptr_t) guardend;
+  if (mprotect (guardend, mprots2, prot) != 0)
+    return errno;
+#endif
+  return 0;
+}
 
 /* Returns a usable stack for a new thread either by allocating a
    new stack or reusing a cached stack of sufficient size.
@@ -490,7 +527,10 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
 	    size += pagesize_m1 + 1;
 #endif
 
-	  mem = mmap (NULL, size, prot,
+	  /* If a guard page is required, avoid committing memory by first
+	     allocate with PROT_NONE and then reserve with required permission
+	     excluding the guard page.  */
+	  mem = mmap (NULL, size, (guardsize == 0) ? prot : PROT_NONE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
 
 	  if (__glibc_unlikely (mem == MAP_FAILED))
@@ -510,9 +550,24 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
 				   - TLS_PRE_TCB_SIZE);
 #endif
 
+	  /* Now mprotect the required region excluding the guard area.  */
+	  if (__glibc_likely (guardsize > 0))
+	    {
+	      char *guard = guard_position (mem, size, guardsize, pd,
+					    pagesize_m1);
+	      if (setup_stack_prot (mem, size, guard, guardsize, prot) != 0)
+		{
+		  munmap (mem, size);
+		  return errno;
+		}
+	    }
+
 	  /* Remember the stack-related values.  */
 	  pd->stackblock = mem;
 	  pd->stackblock_size = size;
+	  /* Update guardsize for newly allocated guardsize to avoid
+	     an mprotect in guard resize below.  */
+	  pd->guardsize = guardsize;
 
 	  /* We allocated the first block thread-specific data array.
 	     This address will not change for the lifetime of this
@@ -593,13 +648,8 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
       /* Create or resize the guard area if necessary.  */
       if (__glibc_unlikely (guardsize > pd->guardsize))
 	{
-#ifdef NEED_SEPARATE_REGISTER_STACK
+	  char *guard = guard_position (mem, size, guardsize, pd,
-	  char *guard = mem + (((size - guardsize) / 2) & ~pagesize_m1);
+					pagesize_m1);
-#elif _STACK_GROWS_DOWN
-	  char *guard = mem;
-#elif _STACK_GROWS_UP
-	  char *guard = (char *) (((uintptr_t) pd - guardsize) & ~pagesize_m1);
-#endif
 	  if (mprotect (guard, guardsize, PROT_NONE) != 0)
 	    {
 	    mprot_error:

support/Makefile

@@ -82,6 +82,7 @@ libsupport-routines = \
   xpthread_attr_init \
   xpthread_attr_setdetachstate \
   xpthread_attr_setstacksize \
+  xpthread_attr_setguardsize \
   xpthread_barrier_destroy \
   xpthread_barrier_init \
   xpthread_barrier_wait \

support/xpthread_attr_setguardsize.c

@@ -0,0 +1,26 @@
+/* pthread_attr_setguardsize with error checking.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xthread.h>
+
+void
+xpthread_attr_setguardsize (pthread_attr_t *attr, size_t guardsize)
+{
+  xpthread_check_return ("pthread_attr_setguardize",
+			 pthread_attr_setguardsize (attr, guardsize));
+}

support/xthread.h

@@ -67,6 +67,8 @@ void xpthread_attr_setdetachstate (pthread_attr_t *attr,
 				   int detachstate);
 void xpthread_attr_setstacksize (pthread_attr_t *attr,
 				 size_t stacksize);
+void xpthread_attr_setguardsize (pthread_attr_t *attr,
+				 size_t guardsize);
 
 /* This function returns non-zero if pthread_barrier_wait returned
    PTHREAD_BARRIER_SERIAL_THREAD.  */