From 0edbf1230131dfeb03d843d2859e2104456fad80 Mon Sep 17 00:00:00 2001 From: Adhemerval Zanella Date: Tue, 31 Jan 2017 18:01:59 -0200 Subject: 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. --- nptl/allocatestack.c | 66 +++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 58 insertions(+), 8 deletions(-) (limited to 'nptl/allocatestack.c') diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c index e5c5f79a82..8364406b1b 100644 --- a/nptl/allocatestack.c +++ b/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 = mem + (((size - guardsize) / 2) & ~pagesize_m1); -#elif _STACK_GROWS_DOWN - char *guard = mem; -#elif _STACK_GROWS_UP - char *guard = (char *) (((uintptr_t) pd - guardsize) & ~pagesize_m1); -#endif + char *guard = guard_position (mem, size, guardsize, pd, + pagesize_m1); if (mprotect (guard, guardsize, PROT_NONE) != 0) { mprot_error: -- cgit 1.4.1