From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-110.freemail.mail.aliyun.com (out30-110.freemail.mail.aliyun.com [115.124.30.110]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C776045C6E5; Thu, 10 Sep 2026 10:44:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.110 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789037101; cv=none; b=NCpIF5lNg7RlmXHTet7vbyRyWE9O7NaNdXGArRm9cEikd8IYqN69y9mwsGXz3SbPOzo0MYjNzO+qkvPVZpx0BVRsNvRzM7jDfqIAKgJMrfXVhwxRff6JL+qu2BIlSiZGh1LOhsXdN1rbBGltMpWtWDjCYy2Kg1F19vbL95gZPWg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789037101; c=relaxed/simple; bh=575mcS9daaIwjeMD0kUFlRZ9z+MakMBNEa3u247RjpU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KVE1UselsWsgEJkDGrIBpuLh7eSBwdsM5tPsNaOXAk4k50chxg4J+miQhGg3IB7QSe1FOSZi63JaC2Nu0c83Ofd5Sj5VgD0Axktl6D8lXpf4/o0nEO5UDri+zdpkZbW4ocQps2Iy/cRjjdfR4JzRnit/O9Q3SIY6ov/VPs38Yyg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=fbYwbJ8E; arc=none smtp.client-ip=115.124.30.110 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="fbYwbJ8E" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1789037090; h=From:To:Subject:Date:Message-ID:MIME-Version; bh=IBlj/dgMk2QlpVlPM12cdYzmxxGpAqdOm7WEKsRRlUE=; b=fbYwbJ8ExyjWakgeBSF6RrrPLaYx+a8IQsIMHzh92szcwUH6lFM1mOhjcEenAgksnFDF0UHHlls9BBG++k39jOFs7Ggx2OqcKBnB/2VEKaV2bA5QhCjY5Erp6bqxf6weICB1ASIw1Idk5zYEDQ+Q47eX7+49wz0asBD0fnO/dPg= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R221e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045098064;MF=alibuda@linux.alibaba.com;NM=1;PH=DS;RN=17;SR=0;TI=SMTPD_---0XAhM9Nl_1789037089; Received: from j66a10360.sqa.eu95.tbsite.net(mailfrom:alibuda@linux.alibaba.com fp:SMTPD_---0XAhM9Nl_1789037089 cluster:ay36) by smtp.aliyun-inc.com; Thu, 10 Sep 2026 18:44:49 +0800 From: "D. Wythe" To: mjambigi@linux.ibm.com, wenjia@linux.ibm.com, wintera@linux.ibm.com, dust.li@linux.alibaba.com, tonylu@linux.alibaba.com, guwen@linux.alibaba.com Cc: kuba@kernel.org, davem@davemloft.net, netdev@vger.kernel.org, linux-s390@vger.kernel.org, linux-rdma@vger.kernel.org, leonro@nvidia.com, pabeni@redhat.com, edumazet@google.com, sidraya@linux.ibm.com, jaka@linux.ibm.com, oliver.yang@linux.alibaba.com Subject: [PATCH net-next 2/2] net/smc: reduce TX slot contention with exclusive wait Date: Thu, 10 Sep 2026 18:44:46 +0800 Message-ID: <20260910104446.41423-3-alibuda@linux.alibaba.com> X-Mailer: git-send-email 2.45.0 In-Reply-To: <20260910104446.41423-1-alibuda@linux.alibaba.com> References: <20260910104446.41423-1-alibuda@linux.alibaba.com> Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit smc_wr_tx_get_free_slot() waits for a free TX slot with wait_event_interruptible_timeout(). Since the wait_event family enqueues waiters as non-exclusive, wake_up() may wake multiple waiters even though only one can use the slot, causing thundering-herd contention when slots are scarce. Use an exclusive wait loop with prepare_to_wait_exclusive() so wake_up() wakes only one waiter per freed slot. smc_wr_wakeup_tx_wait() still uses wake_up_all() during link teardown, so teardown behavior is unchanged. This also corrects the return value on a pending signal: the previous wait_event_interruptible_timeout() path fell through to the "no free slot" case and returned -EPIPE, masking the signal as a connection error. The open-coded loop now returns -ERESTARTSYS, matching the standard interruptible-wait semantics and letting the syscall restart machinery handle it. Performance =========== Measured with uperf between two peers over SMC-R. The benefit depends on how often the TX slot wait path is actually taken. With the default settings, where many connections share a link group and the send queue is small, slots are scarce and the wait path is hot: net.smc.smcr_max_conns_per_lgr = 255 net.smc.smcr_max_send_wr = 16 net.smc.smcr_max_recv_wr = 48 workload baseline patched delta --------------------------------------------------------- rr1c-200x1000-50.xml 655.06 Mb/s 1.53 Gb/s +134% rr1c-1x1-250.xml 371.03 Kb/s 2.07 Mb/s +458% With a tuned configuration, where slots are mostly available and the wait path is rarely entered, the change is neutral to slightly positive: net.smc.smcr_max_conns_per_lgr = 32 net.smc.smcr_max_send_wr = 64 net.smc.smcr_max_recv_wr = 64 workload baseline patched delta --------------------------------------------------------- rr1c-200x1000-50.xml 1.74 Gb/s 1.80 Gb/s +3.4% rr1c-1x1-250.xml 3.11 Mb/s 3.14 Mb/s +1.0% So the change does not regress the uncontended case, and recovers most of the throughput lost to thundering-herd wakeups once slots become scarce. Signed-off-by: D. Wythe Reviewed-by: Wen Gu Reviewed-by: Mahanta Jambigi --- net/smc/smc_wr.c | 36 ++++++++++++++++++++++++++---------- 1 file changed, 26 insertions(+), 10 deletions(-) diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c index def2ab84b0c7..64413008f9c1 100644 --- a/net/smc/smc_wr.c +++ b/net/smc/smc_wr.c @@ -198,11 +198,13 @@ int smc_wr_tx_get_free_slot(struct smc_link *link, struct smc_rdma_wr **wr_rdma_buf, struct smc_wr_tx_pend_priv **wr_pend_priv) { + unsigned long timeout = SMC_WR_TX_WAIT_FREE_SLOT_TIME; struct smc_link_group *lgr = smc_get_lgr(link); struct smc_wr_tx_pend *wr_pend; u32 idx = link->wr_tx_cnt; struct ib_send_wr *wr_ib; u64 wr_id; + DEFINE_WAIT(wait); int rc; *wr_buf = NULL; @@ -212,17 +214,31 @@ int smc_wr_tx_get_free_slot(struct smc_link *link, if (rc) return rc; } else { - rc = wait_event_interruptible_timeout( - link->wr_tx_wait, - !smc_link_sendable(link) || - lgr->terminating || - (smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY), - SMC_WR_TX_WAIT_FREE_SLOT_TIME); - if (!rc) { - /* timeout - terminate link */ - smcr_link_down_cond_sched(link); - return -EPIPE; + rc = 0; + for (;;) { + prepare_to_wait_exclusive(&link->wr_tx_wait, &wait, + TASK_INTERRUPTIBLE); + if (!smc_link_sendable(link) || lgr->terminating || + smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY) + break; + timeout = schedule_timeout(timeout); + /* re-check */ + if (!smc_link_sendable(link) || lgr->terminating || + smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY) + break; + if (!timeout) { + /* timeout - terminate link */ + smcr_link_down_cond_sched(link); + break; + } + if (signal_pending(current)) { + rc = -ERESTARTSYS; + break; + } } + finish_wait(&link->wr_tx_wait, &wait); + if (rc) + return rc; if (idx == link->wr_tx_cnt) return -EPIPE; } -- 2.45.0