Commit 0cba6442 authored by Anish Moorthy's avatar Anish Moorthy Committed by Sean Christopherson
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KVM: selftests: Use EPOLL in userfaultfd_util reader threads 



With multiple reader threads POLLing a single UFFD, the demand paging 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, i.e. 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: default avatarAnish Moorthy <amoorthy@google.com>
Acked-by: default avatarJames Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20240215235405.368539-13-amoorthy@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent df4ec5aa

tools/testing/selftests/kvm/demand_paging_test.c

+0 −1
Original line number Diff line number Diff line
@@ -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>

tools/testing/selftests/kvm/lib/userfaultfd_util.c

+35 −39
Original line number Diff line number Diff line
@@ -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;

	struct epoll_event evt;

	int epollfd;



	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),

		    "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),

		    "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");

				    "Error reading pipefd in uffd reader thread");

			break;

		}



		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)

			TEST_ASSERT(errno == EAGAIN,

				    "Error reading from UFFD: errno = %d", errno);

			continue;

			pr_info("Read of uffd got errno %d\n", errno);

			return NULL;

		}



		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++;

	}