From: Muchun Song songmuchun@bytedance.com
[ Upstream commit 14c24048841151548a3f4d9e218510c844c1b737 ]
We found that a process with 10 thousnads threads has been encountered a regression problem from Linux-v4.14 to Linux-v5.4. It is a kind of workload which will concurrently allocate lots of memory in different threads sometimes. In this case, we will see the down_read_trylock() with a high hotspot. Therefore, we suppose that rwsem has a regression at least since Linux-v5.4. In order to easily debug this problem, we write a simply benchmark to create the similar situation lile the following.
```c++ #include <sys/mman.h> #include <sys/time.h> #include <sys/resource.h> #include <sched.h>
#include <cstdio> #include <cassert> #include <thread> #include <vector> #include <chrono>
volatile int mutex;
void trigger(int cpu, char* ptr, std::size_t sz) { cpu_set_t set; CPU_ZERO(&set); CPU_SET(cpu, &set); assert(pthread_setaffinity_np(pthread_self(), sizeof(set), &set) == 0);
while (mutex);
for (std::size_t i = 0; i < sz; i += 4096) { *ptr = '\0'; ptr += 4096; } }
int main(int argc, char* argv[]) { std::size_t sz = 100;
if (argc > 1) sz = atoi(argv[1]);
auto nproc = std::thread::hardware_concurrency(); std::vectorstd::thread thr; sz <<= 30; auto* ptr = mmap(nullptr, sz, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0); assert(ptr != MAP_FAILED); char* cptr = static_cast<char*>(ptr); auto run = sz / nproc; run = (run >> 12) << 12;
mutex = 1;
for (auto i = 0U; i < nproc; ++i) { thr.emplace_back(std::thread([i, cptr, run]() { trigger(i, cptr, run); })); cptr += run; }
rusage usage_start; getrusage(RUSAGE_SELF, &usage_start); auto start = std::chrono::system_clock::now();
mutex = 0;
for (auto& t : thr) t.join();
rusage usage_end; getrusage(RUSAGE_SELF, &usage_end); auto end = std::chrono::system_clock::now(); timeval utime; timeval stime; timersub(&usage_end.ru_utime, &usage_start.ru_utime, &utime); timersub(&usage_end.ru_stime, &usage_start.ru_stime, &stime); printf("usr: %ld.%06ld\n", utime.tv_sec, utime.tv_usec); printf("sys: %ld.%06ld\n", stime.tv_sec, stime.tv_usec); printf("real: %lu\n", std::chrono::duration_caststd::chrono::milliseconds(end - start).count());
return 0; } ```
The functionality of above program is simply which creates `nproc` threads and each of them are trying to touch memory (trigger page fault) on different CPU. Then we will see the similar profile by `perf top`.
25.55% [kernel] [k] down_read_trylock 14.78% [kernel] [k] handle_mm_fault 13.45% [kernel] [k] up_read 8.61% [kernel] [k] clear_page_erms 3.89% [kernel] [k] __do_page_fault
The highest hot instruction, which accounts for about 92%, in down_read_trylock() is cmpxchg like the following.
91.89 │ lock cmpxchg %rdx,(%rdi)
Sice the problem is found by migrating from Linux-v4.14 to Linux-v5.4, so we easily found that the commit ddb20d1d3aed ("locking/rwsem: Optimize down_read_trylock()") caused the regression. The reason is that the commit assumes the rwsem is not contended at all. But it is not always true for mmap lock which could be contended with thousands threads. So most threads almost need to run at least 2 times of "cmpxchg" to acquire the lock. The overhead of atomic operation is higher than non-atomic instructions, which caused the regression.
By using the above benchmark, the real executing time on a x86-64 system before and after the patch were:
Before Patch After Patch # of Threads real real reduced by ------------ ------ ------ ---------- 1 65,373 65,206 ~0.0% 4 15,467 15,378 ~0.5% 40 6,214 5,528 ~11.0%
For the uncontended case, the new down_read_trylock() is the same as before. For the contended cases, the new down_read_trylock() is faster than before. The more contended, the more fast.
Signed-off-by: Muchun Song songmuchun@bytedance.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Waiman Long longman@redhat.com Link: https://lore.kernel.org/r/20211118094455.9068-1-songmuchun@bytedance.com Stable-dep-of: 3f5245538a19 ("locking/rwsem: Disable preemption in all down_read*() and up_read() code paths") Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/locking/rwsem.c | 11 ++++------- 1 file changed, 4 insertions(+), 7 deletions(-)
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 4cc73e6f8974b..73cff464dde00 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -1255,17 +1255,14 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
- /* - * Optimize for the case when the rwsem is not locked at all. - */ - tmp = RWSEM_UNLOCKED_VALUE; - do { + tmp = atomic_long_read(&sem->count); + while (!(tmp & RWSEM_READ_FAILED_MASK)) { if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, - tmp + RWSEM_READER_BIAS)) { + tmp + RWSEM_READER_BIAS)) { rwsem_set_reader_owned(sem); return 1; } - } while (!(tmp & RWSEM_READ_FAILED_MASK)); + } return 0; }