Re: [PATCH] tap: Send frames after the first one in tap_send_frames_pasta()

On Mon, Feb 13, 2023 at 02:12:11AM +0100, Stefano Brivio wrote:

...instead of repeatedly sending out the first one in iov.

Fixes: e21ee41ac35a ("tcp: Combine two parts of pasta tap send path together") Signed-off-by: Stefano Brivio --- I just applied this, to unblock a series by David which was pending for way too long. The commit reference in Fixes: refers to a commit from said series which I'm pushing out together with this patch.

Huh... how did this ever work even slightly. From that point of view, Reviewed-by: David Gibson

Posting anyway for reviews.

That said..

tap.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tap.c b/tap.c index af9bc15..716d887 100644 --- a/tap.c +++ b/tap.c @@ -316,12 +316,13 @@ static void tap_send_frames_pasta(struct ctx *c, { size_t i;

- for (i = 0; i < n; i++) { + for (i = 0; i < n; i++, iov++) {

I quite dislike having multiple "counters" that need to be updated for each loop iteration (manual strength reduction. It's really easy to make a mistake in later changes and let the two values get out of sync - which is exactly what I did with the earlier change that introduced this bug. W.r.t. performance, I generally trust the compiler's automatic strength reduction to have a better idea of whether it will be worth it or not than my own guess.

if (write(c->fd_tap, (char *)iov->iov_base, iov->iov_len) < 0) {

So, my *intention* on the older patch was to replace 'iov->' above with 'iov[i].'

debug("tap write: %s", strerror(errno)); if (errno != EAGAIN && errno != EWOULDBLOCK) tap_handler(c, c->fd_tap, EPOLLERR, NULL); i--; + iov--; } } }

-- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson