With multiple reader threads POLLing a single UFFD, the test suffers
from the thundering herd problem: performance degrades as the number of
reader threads is increased. Solve this issue [1] by switching the
the polling mechanism to EPOLL + EPOLLEXCLUSIVE.
Also, change the error-handling convention of uffd_handler_thread_fn.
Instead of just printing errors and returning early from the polling
loop, check for them via TEST_ASSERT. "return NULL" is reserved for a
successful exit from uffd_handler_thread_fn, ie one triggered by a
write to the exit pipe.
Performance samples generated by the command in [2] are given below.
Num Reader Threads, Paging Rate (POLL), Paging Rate (EPOLL)
1 249k 185k
2 201k 235k
4 186k 155k
16 150k 217k
32 89k 198k
[1] Single-vCPU performance does suffer somewhat.
[2] ./demand_paging_test -u MINOR -s shmem -v 4 -o -r <num readers>
Signed-off-by: Anish Moorthy <amoorthy@xxxxxxxxxx>
Acked-by: James Houghton <jthoughton@xxxxxxxxxx>
---
.../selftests/kvm/demand_paging_test.c | 1 -
.../selftests/kvm/lib/userfaultfd_util.c | 74 +++++++++----------
2 files changed, 35 insertions(+), 40 deletions(-)
diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
index 6c2253f4a64ef..c729cee4c2055 100644
--- a/tools/testing/selftests/kvm/demand_paging_test.c
+++ b/tools/testing/selftests/kvm/demand_paging_test.c
@@ -13,7 +13,6 @@
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
-#include <poll.h>
#include <pthread.h>
#include <linux/userfaultfd.h>
#include <sys/syscall.h>
diff --git a/tools/testing/selftests/kvm/lib/userfaultfd_util.c b/tools/testing/selftests/kvm/lib/userfaultfd_util.c
index 2723ee1e3e1b2..909ad69c1cb04 100644
--- a/tools/testing/selftests/kvm/lib/userfaultfd_util.c
+++ b/tools/testing/selftests/kvm/lib/userfaultfd_util.c
@@ -16,6 +16,7 @@
#include <poll.h>
#include <pthread.h>
#include <linux/userfaultfd.h>
+#include <sys/epoll.h>
#include <sys/syscall.h>
#include "kvm_util.h"
@@ -32,60 +33,55 @@ static void *uffd_handler_thread_fn(void *arg)
int64_t pages = 0;
struct timespec start;
struct timespec ts_diff;
+ int epollfd;
+ struct epoll_event evt;
+
+ epollfd = epoll_create(1);
+ TEST_ASSERT(epollfd >= 0, "Failed to create epollfd.");
+
+ evt.events = EPOLLIN | EPOLLEXCLUSIVE;
+ evt.data.u32 = 0;
+ TEST_ASSERT(epoll_ctl(epollfd, EPOLL_CTL_ADD, uffd, &evt) == 0,
+ "Failed to add uffd to epollfd");
+
+ evt.events = EPOLLIN;
+ evt.data.u32 = 1;
+ TEST_ASSERT(epoll_ctl(epollfd, EPOLL_CTL_ADD, reader_args->pipe, &evt) == 0,
+ "Failed to add pipe to epollfd");
clock_gettime(CLOCK_MONOTONIC, &start);
while (1) {
struct uffd_msg msg;
- struct pollfd pollfd[2];
- char tmp_chr;
int r;
- pollfd[0].fd = uffd;
- pollfd[0].events = POLLIN;
- pollfd[1].fd = reader_args->pipe;
- pollfd[1].events = POLLIN;
-
- r = poll(pollfd, 2, -1);
- switch (r) {
- case -1:
- pr_info("poll err");
- continue;
- case 0:
- continue;
- case 1:
- break;
- default:
- pr_info("Polling uffd returned %d", r);
- return NULL;
- }
+ r = epoll_wait(epollfd, &evt, 1, -1);
+ TEST_ASSERT(r == 1,
+ "Unexpected number of events (%d) from epoll, errno = %d",
+ r, errno);
- if (pollfd[0].revents & POLLERR) {
- pr_info("uffd revents has POLLERR");
- return NULL;
- }
+ if (evt.data.u32 == 1) {
+ char tmp_chr;
- if (pollfd[1].revents & POLLIN) {
- r = read(pollfd[1].fd, &tmp_chr, 1);
+ TEST_ASSERT(!(evt.events & (EPOLLERR | EPOLLHUP)),
+ "Reader thread received EPOLLERR or EPOLLHUP on pipe.");
+ r = read(reader_args->pipe, &tmp_chr, 1);
TEST_ASSERT(r == 1,
- "Error reading pipefd in UFFD thread\n");
+ "Error reading pipefd in uffd reader thread");
return NULL;
}
- if (!(pollfd[0].revents & POLLIN))
- continue;
+ TEST_ASSERT(!(evt.events & (EPOLLERR | EPOLLHUP)),
+ "Reader thread received EPOLLERR or EPOLLHUP on uffd.");
r = read(uffd, &msg, sizeof(msg));
if (r == -1) {
- if (errno == EAGAIN)
- continue;
- pr_info("Read of uffd got errno %d\n", errno);
- return NULL;
+ TEST_ASSERT(errno == EAGAIN,
+ "Error reading from UFFD: errno = %d", errno);
+ continue;
}
- if (r != sizeof(msg)) {
- pr_info("Read on uffd returned unexpected size: %d bytes", r);
- return NULL;
- }
+ TEST_ASSERT(r == sizeof(msg),
+ "Read on uffd returned unexpected number of bytes (%d)", r);
if (!(msg.event & UFFD_EVENT_PAGEFAULT))
continue;
@@ -93,8 +89,8 @@ static void *uffd_handler_thread_fn(void *arg)
if (reader_args->delay)
usleep(reader_args->delay);
r = reader_args->handler(reader_args->uffd_mode, uffd, &msg);
- if (r < 0)
- return NULL;
+ TEST_ASSERT(r >= 0,
+ "Reader thread handler fn returned negative value %d", r);
pages++;
}
--
2.40.0.577.gac1e443424-goog
