Re: [PATCH infiniband-diags] query_smp.c: Avoid busy looping in process_one_recv

ira.weiny
2015-04-22T14:44:43Z

On Thu, Apr 16, 2015 at 10:26:24AM -0400, Hal Rosenstock wrote:
> From: Vladimir Koushnir <vladimirk-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
> Date: Thu, 16 Apr 2015 14:11:16 +0300
>
> Today, process_one_recv calls umad_recv with zero timeout.
> That means umad_recv will use non-blocking read() without polling,
> In many cases, this causes busy loop.
>
> In the fix, umad_recv is invoked with -1 (infinite timeout) and the read()
> will occur only when poll exits. Also, no longer need to check for
> EWOULDBLOCK.

Has this been tested on broken fabrics where SMAs don't respond?

I know that we should recv something (ie timed out Send) for every MAD
submitted to the kernel but I seem to remember a reason we did not use a
timeout. But that may have been for a previous implementation which no longer
exists. I can't remember.

Ira

>
> Signed-off-by: Vladimir Koushnir <vladimirk-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
> Signed-off-by: Hal Rosenstock <hal-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
> ---
> libibnetdisc/src/query_smp.c | 2 +-
> 1 files changed, 1 insertions(+), 3 deletions(-)
>
> diff --git a/libibnetdisc/src/query_smp.c b/libibnetdisc/src/query_smp.c
> index 28620b4..8f179e4 100644
> --- a/libibnetdisc/src/query_smp.c
> +++ b/libibnetdisc/src/query_smp.c
> @@ -171,9 +171,7 @@ static int process_one_recv(smp_engine_t * engine)
>
> /* wait for the next message */
> if ((rc = umad_recv(engine->umad_fd, umad, &length,
> - 0)) < 0) {
> - if (rc == -EWOULDBLOCK)
> - return 0;
> + -1)) < 0) {
> IBND_ERROR("umad_recv failed: %d\n", rc);
> return -1;
> }