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Date: Fri, 15 Nov 2019 21:07:46 -0500
From: Soheil Hassas Yeganeh <soheil@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
Eric Dumazet <eric.dumazet@...il.com>,
Arjun Roy <arjunroy@...gle.com>
Subject: Re: [PATCH net-next] selftests: net: avoid ptl lock contention in tcp_mmap
On Fri, Nov 15, 2019 at 8:56 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> tcp_mmap is used as a reference program for TCP rx zerocopy,
> so it is important to point out some potential issues.
>
> If multiple threads are concurrently using getsockopt(...
> TCP_ZEROCOPY_RECEIVE), there is a chance the low-level mm
> functions compete on shared ptl lock, if vma are arbitrary placed.
>
> Instead of letting the mm layer place the chunks back to back,
> this patch enforces an alignment so that each thread uses
> a different ptl lock.
>
> Performance measured on a 100 Gbit NIC, with 8 tcp_mmap clients
> launched at the same time :
>
> $ for f in {1..8}; do ./tcp_mmap -H 2002:a05:6608:290:: & done
>
> In the following run, we reproduce the old behavior by requesting no alignment :
>
> $ tcp_mmap -sz -C $((128*1024)) -a 4096
> received 32768 MB (100 % mmap'ed) in 9.69532 s, 28.3516 Gbit
> cpu usage user:0.08634 sys:3.86258, 120.511 usec per MB, 171839 c-switches
> received 32768 MB (100 % mmap'ed) in 25.4719 s, 10.7914 Gbit
> cpu usage user:0.055268 sys:21.5633, 659.745 usec per MB, 9065 c-switches
> received 32768 MB (100 % mmap'ed) in 28.5419 s, 9.63069 Gbit
> cpu usage user:0.057401 sys:23.8761, 730.392 usec per MB, 14987 c-switches
> received 32768 MB (100 % mmap'ed) in 28.655 s, 9.59268 Gbit
> cpu usage user:0.059689 sys:23.8087, 728.406 usec per MB, 18509 c-switches
> received 32768 MB (100 % mmap'ed) in 28.7808 s, 9.55074 Gbit
> cpu usage user:0.066042 sys:23.4632, 718.056 usec per MB, 24702 c-switches
> received 32768 MB (100 % mmap'ed) in 28.8259 s, 9.5358 Gbit
> cpu usage user:0.056547 sys:23.6628, 723.858 usec per MB, 23518 c-switches
> received 32768 MB (100 % mmap'ed) in 28.8808 s, 9.51767 Gbit
> cpu usage user:0.059357 sys:23.8515, 729.703 usec per MB, 14691 c-switches
> received 32768 MB (100 % mmap'ed) in 28.8879 s, 9.51534 Gbit
> cpu usage user:0.047115 sys:23.7349, 725.769 usec per MB, 21773 c-switches
>
> New behavior (automatic alignment based on Hugepagesize),
> we can see the system overhead being dramatically reduced.
>
> $ tcp_mmap -sz -C $((128*1024))
> received 32768 MB (100 % mmap'ed) in 13.5339 s, 20.3103 Gbit
> cpu usage user:0.122644 sys:3.4125, 107.884 usec per MB, 168567 c-switches
> received 32768 MB (100 % mmap'ed) in 16.0335 s, 17.1439 Gbit
> cpu usage user:0.132428 sys:3.55752, 112.608 usec per MB, 188557 c-switches
> received 32768 MB (100 % mmap'ed) in 17.5506 s, 15.6621 Gbit
> cpu usage user:0.155405 sys:3.24889, 103.891 usec per MB, 226652 c-switches
> received 32768 MB (100 % mmap'ed) in 19.1924 s, 14.3222 Gbit
> cpu usage user:0.135352 sys:3.35583, 106.542 usec per MB, 207404 c-switches
> received 32768 MB (100 % mmap'ed) in 22.3649 s, 12.2906 Gbit
> cpu usage user:0.142429 sys:3.53187, 112.131 usec per MB, 250225 c-switches
> received 32768 MB (100 % mmap'ed) in 22.5336 s, 12.1986 Gbit
> cpu usage user:0.140654 sys:3.61971, 114.757 usec per MB, 253754 c-switches
> received 32768 MB (100 % mmap'ed) in 22.5483 s, 12.1906 Gbit
> cpu usage user:0.134035 sys:3.55952, 112.718 usec per MB, 252997 c-switches
> received 32768 MB (100 % mmap'ed) in 22.6442 s, 12.139 Gbit
> cpu usage user:0.126173 sys:3.71251, 117.147 usec per MB, 253728 c-switches
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Neat idea! Thank you for the patch!
> Cc: Arjun Roy <arjunroy@...gle.com>
> ---
> tools/testing/selftests/net/tcp_mmap.c | 58 +++++++++++++++++++++++---
> 1 file changed, 53 insertions(+), 5 deletions(-)
>
> diff --git a/tools/testing/selftests/net/tcp_mmap.c b/tools/testing/selftests/net/tcp_mmap.c
> index 0e73a30f0c2262e62a5ed1e2db6c7c8977bf44fa..5bb370a0857ec8a24916f583be5374183a9aefc8 100644
> --- a/tools/testing/selftests/net/tcp_mmap.c
> +++ b/tools/testing/selftests/net/tcp_mmap.c
> @@ -82,7 +82,9 @@ static int zflg; /* zero copy option. (MSG_ZEROCOPY for sender, mmap() for recei
> static int xflg; /* hash received data (simple xor) (-h option) */
> static int keepflag; /* -k option: receiver shall keep all received file in memory (no munmap() calls) */
>
> -static int chunk_size = 512*1024;
> +static size_t chunk_size = 512*1024;
> +
> +static size_t map_align;
>
> unsigned long htotal;
>
> @@ -118,6 +120,9 @@ void hash_zone(void *zone, unsigned int length)
> htotal = temp;
> }
>
> +#define ALIGN_UP(x, align_to) (((x) + ((align_to)-1)) & ~((align_to)-1))
> +#define ALIGN_PTR_UP(p, ptr_align_to) ((typeof(p))ALIGN_UP((unsigned long)(p), ptr_align_to))
> +
> void *child_thread(void *arg)
> {
> unsigned long total_mmap = 0, total = 0;
> @@ -126,6 +131,7 @@ void *child_thread(void *arg)
> int flags = MAP_SHARED;
> struct timeval t0, t1;
> char *buffer = NULL;
> + void *raddr = NULL;
> void *addr = NULL;
> double throughput;
> struct rusage ru;
> @@ -142,9 +148,13 @@ void *child_thread(void *arg)
> goto error;
> }
> if (zflg) {
> - addr = mmap(NULL, chunk_size, PROT_READ, flags, fd, 0);
> - if (addr == (void *)-1)
> + raddr = mmap(NULL, chunk_size + map_align, PROT_READ, flags, fd, 0);
> + if (raddr == (void *)-1) {
> + perror("mmap");
> zflg = 0;
> + } else {
> + addr = ALIGN_PTR_UP(raddr, map_align);
> + }
> }
> while (1) {
> struct pollfd pfd = { .fd = fd, .events = POLLIN, };
> @@ -222,7 +232,7 @@ void *child_thread(void *arg)
> free(buffer);
> close(fd);
> if (zflg)
> - munmap(addr, chunk_size);
> + munmap(raddr, chunk_size + map_align);
> pthread_exit(0);
> }
>
> @@ -303,6 +313,30 @@ static void do_accept(int fdlisten)
> }
> }
>
> +/* Each thread should reserve a big enough vma to avoid
> + * spinlock collisions in ptl locks.
> + * This size is 2MB on x86_64, and is exported in /proc/meminfo.
> + */
> +static unsigned long default_huge_page_size(void)
> +{
> + FILE *f = fopen("/proc/meminfo", "r");
> + unsigned long hps = 0;
> + size_t linelen = 0;
> + char *line = NULL;
> +
> + if (!f)
> + return 0;
> + while (getline(&line, &linelen, f) > 0) {
> + if (sscanf(line, "Hugepagesize: %lu kB", &hps) == 1) {
> + hps <<= 10;
> + break;
> + }
> + }
> + free(line);
> + fclose(f);
> + return hps;
> +}
> +
> int main(int argc, char *argv[])
> {
> struct sockaddr_storage listenaddr, addr;
> @@ -314,7 +348,7 @@ int main(int argc, char *argv[])
> int sflg = 0;
> int mss = 0;
>
> - while ((c = getopt(argc, argv, "46p:svr:w:H:zxkP:M:")) != -1) {
> + while ((c = getopt(argc, argv, "46p:svr:w:H:zxkP:M:C:a:")) != -1) {
> switch (c) {
> case '4':
> cfg_family = PF_INET;
> @@ -354,10 +388,24 @@ int main(int argc, char *argv[])
> case 'P':
> max_pacing_rate = atoi(optarg) ;
> break;
> + case 'C':
> + chunk_size = atol(optarg);
> + break;
> + case 'a':
> + map_align = atol(optarg);
> + break;
> default:
> exit(1);
> }
> }
> + if (!map_align) {
> + map_align = default_huge_page_size();
> + /* if really /proc/meminfo is not helping,
> + * we use the default x86_64 hugepagesize.
> + */
> + if (!map_align)
> + map_align = 2*1024*1024;
> + }
> if (sflg) {
> int fdlisten = socket(cfg_family, SOCK_STREAM, 0);
>
> --
> 2.24.0.432.g9d3f5f5b63-goog
>
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