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Christian McDonald Subject: git: bf9ac4f14081 - stable/15 - iflib: Do not ring the transmit doorbell when nothing is pending List-Archive: https://lists.freebsd.org/archives/dev-commits-src-branches X-BeenThere: dev-commits-src-branches@freebsd.org Sender: owner-dev-commits-src-branches@FreeBSD.org List-Id: Commits to the stable branches of the FreeBSD src repository List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Git-Committer: rcm X-Git-Repository: src X-Git-Refname: refs/heads/stable/15 X-Git-Reftype: branch X-Git-Commit: bf9ac4f140815520451a5581eca888f88124f3e7 Auto-Submitted: auto-generated Date: Wed, 28 Oct 2026 20:35:48 +0000 Message-Id: <6ae25ca4.1d61a.61972d14@gitrepo.freebsd.org> The branch stable/15 has been updated by rcm: URL: https://cgit.FreeBSD.org/src/commit/?id=bf9ac4f140815520451a5581eca888f88124f3e7 commit bf9ac4f140815520451a5581eca888f88124f3e7 Author: R. Christian McDonald AuthorDate: 2026-10-05 15:17:13 +0000 Commit: R. Christian McDonald CommitDate: 2026-10-28 20:34:09 +0000 iflib: Do not ring the transmit doorbell when nothing is pending For a lightly used ring iflib_txd_db_check() may defer zero descriptors, so its "pending >= limit" test is true even when nothing has been queued since the last doorbell. iflib_txq_drain() calls it before, inside and after its loop, so a sender that drains its own packet wrote the tail register three times per packet, twice with the value the hardware already had. The log of 81be655266fa ("iflib: ensure that tx interrupts enabled and cleanups") calls skipping the doorbell when db_pending is zero "an obvious missing optimization"; the comparison against a limit of zero defeated it. vmx(4) and mgb(4) have dropped such repeated requests in the driver since 2019. Return early when nothing is pending. Reviewed by: gallatin MFC after: 2 weeks Sponsored by: Rubicon Communications, LLC ("Netgate") Differential Revision: https://reviews.freebsd.org/D60290 (cherry picked from commit 4a0ec469934a5de60b9b540689703a698ac44105) --- sys/net/iflib.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/sys/net/iflib.c b/sys/net/iflib.c index eec7c48f274c..c0085156e385 100644 --- a/sys/net/iflib.c +++ b/sys/net/iflib.c @@ -3072,6 +3072,13 @@ iflib_txd_db_check(iflib_txq_t txq, int ring) if_ctx_t ctx = txq->ift_ctx; qidx_t dbval, max; + /* + * Nothing has been queued since the last doorbell. The threshold is + * zero for a lightly used ring, so the test below would not catch this. + */ + if (txq->ift_db_pending == 0) + return (false); + max = TXQ_MAX_DB_DEFERRED(txq, txq->ift_in_use); /* force || threshold exceeded || at the edge of the ring */