Re: [PATCH] Created tunable to force small pages on stack allocation.

[Cupertino Miranda <cupertino.miranda@oracle.com>]

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> I suspect that quite a few failing madvise calls are cheaper than the
> those three system calls in __malloc_thp_mode.  In addition,
> __malloc_thp_mode may fail due to future kernel changes, disabling the
> tunable by accident.

Please find the new patch which removes the __malloc_thp_mode
verification below.
Looking forward to your comments.

Cupertino


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From a4c2392d52ae9864fb0a602a3959c9f6a39714e4 Mon Sep 17 00:00:00 2001
From: Cupertino Miranda <cupertino.miranda@oracle.com>
Date: Mon, 6 Mar 2023 19:34:19 +0000
Subject: [PATCH] Created tunable to force small pages on stack allocation.

Created tunable glibc.pthread.stack_hugetlb to control when hugepages
can be used for stack allocation.
In case THP are enabled and glibc.pthread.stack_hugetlb is set to
0, glibc will madvise the kernel not to use allow hugepages for stack
allocations.
---
 nptl/allocatestack.c          | 6 ++++++
 nptl/nptl-stack.c             | 1 +
 nptl/nptl-stack.h             | 3 +++
 nptl/pthread_mutex_conf.c     | 8 ++++++++
 sysdeps/nptl/dl-tunables.list | 6 ++++++
 5 files changed, 24 insertions(+)

diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
index c7adbccd6f..c792c6ed1f 100644
--- a/nptl/allocatestack.c
+++ b/nptl/allocatestack.c
@@ -369,6 +369,12 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
 	  if (__glibc_unlikely (mem == MAP_FAILED))
 	    return errno;
 
+	  /* Do madvise in case the tunable glibc.pthread.stack_hugetlb is
+	     set to 0, disabling hugetlb.  */
+	  if (__glibc_unlikely (__nptl_stack_hugetlb == 0)
+	      && __madvise(mem, size, MADV_NOHUGEPAGE) != 0)
+	    return errno;
+
 	  /* SIZE is guaranteed to be greater than zero.
 	     So we can never get a null pointer back from mmap.  */
 	  assert (mem != NULL);
diff --git a/nptl/nptl-stack.c b/nptl/nptl-stack.c
index 5eb7773575..e829711cb5 100644
--- a/nptl/nptl-stack.c
+++ b/nptl/nptl-stack.c
@@ -21,6 +21,7 @@
 #include <pthreadP.h>
 
 size_t __nptl_stack_cache_maxsize = 40 * 1024 * 1024;
+int32_t __nptl_stack_hugetlb = 1;
 
 void
 __nptl_stack_list_del (list_t *elem)
diff --git a/nptl/nptl-stack.h b/nptl/nptl-stack.h
index 34f8bbb15e..d5b2612b4a 100644
--- a/nptl/nptl-stack.h
+++ b/nptl/nptl-stack.h
@@ -27,6 +27,9 @@
 /* Maximum size of the cache, in bytes.  40 MiB by default.  */
 extern size_t __nptl_stack_cache_maxsize attribute_hidden;
 
+/* Should allow stacks to use hugetlb. 1 is default */
+extern int32_t __nptl_stack_hugetlb;
+
 /* Check whether the stack is still used or not.  */
 static inline bool
 __nptl_stack_in_use (struct pthread *pd)
diff --git a/nptl/pthread_mutex_conf.c b/nptl/pthread_mutex_conf.c
index 329c4cbb8f..60ef9095aa 100644
--- a/nptl/pthread_mutex_conf.c
+++ b/nptl/pthread_mutex_conf.c
@@ -45,6 +45,12 @@ TUNABLE_CALLBACK (set_stack_cache_size) (tunable_val_t *valp)
   __nptl_stack_cache_maxsize = valp->numval;
 }
 
+static void
+TUNABLE_CALLBACK (set_stack_hugetlb) (tunable_val_t *valp)
+{
+  __nptl_stack_hugetlb = (int32_t) valp->numval;
+}
+
 void
 __pthread_tunables_init (void)
 {
@@ -52,5 +58,7 @@ __pthread_tunables_init (void)
                TUNABLE_CALLBACK (set_mutex_spin_count));
   TUNABLE_GET (stack_cache_size, size_t,
                TUNABLE_CALLBACK (set_stack_cache_size));
+  TUNABLE_GET (stack_hugetlb, int32_t,
+	       TUNABLE_CALLBACK (set_stack_hugetlb));
 }
 #endif
diff --git a/sysdeps/nptl/dl-tunables.list b/sysdeps/nptl/dl-tunables.list
index bd1ddb121d..22fa9e0d12 100644
--- a/sysdeps/nptl/dl-tunables.list
+++ b/sysdeps/nptl/dl-tunables.list
@@ -33,5 +33,11 @@ glibc {
       maxval: 1
       default: 1
     }
+    stack_hugetlb {
+      type: INT_32
+      minval: 0
+      maxval: 1
+      default: 0
+    }
   }
 }
-- 
2.38.1