From nobody Fri Dec 19 14:23:46 2025 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 0D67831CA7B for ; Wed, 5 Nov 2025 19:06:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762369604; cv=none; b=nmCbbICHwqbIjA8/hy+pY64fGkTfhNm0b81gBboRID7e+Gsjp2oFZd/BvTRAnSU3+hDPz33kWLgPlrQbikieqvskfDTzRP+ajFvU1w3FoTFlYNEdqBp/hfJUAsWhJ0jyyWg/1Gw49DB18ON78bzITJxD7M+iOY4faJv7UbpgT/o= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762369604; c=relaxed/simple; bh=RYe/If3PsHwZz/rCLCuzwgwWY7iLmzRWSdIDURVa7A4=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=Z/3WvhtEaQLXuMDO6zzeghEtiCAWrmZAYtY8AcAjLxgVfX1nhDTUq9oHuqdtXE2WjnwjM9bOTfnGU/KkxnfradAsXaZQ88MTZqoKQb4SmjWo6+RjLO+kN4h3P3p9+1RvP9BusnPvaJVp3Qn8Nd718YuXIPTo2sfFK0eEsQ7trE4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=QQtE0+9W; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="QQtE0+9W" Received: from DESKTOP-0403QTC.corp.microsoft.com (unknown [20.236.11.42]) by linux.microsoft.com (Postfix) with ESMTPSA id 0AA3C211E327; Wed, 5 Nov 2025 11:06:41 -0800 (PST) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 0AA3C211E327 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1762369602; bh=EFJ4UdqVGukQrPXaao8kzmaswKZEei/7I1BdA506Z2U=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=QQtE0+9W4fJq2vYBUgq6ZZFjrdFkBx/7x0Gb9Rq+oOLnDfDU8UYQt5FmVkMOnosPh Fip0Jd2ML7UQowF5GNbe0XZ/IJUeaD5vDyjoDa5LySJcs2WPoS9o/Wey/DoSeTlpuZ 0AUecMQsfRzhmqo5RqupKXx8N/LHOqg1wbyPPGPg= From: Jacob Pan To: linux-kernel@vger.kernel.org, "iommu@lists.linux.dev" , Will Deacon , Joerg Roedel , Mostafa Saleh , Jason Gunthorpe , Robin Murphy , Nicolin Chen Cc: Jacob Pan , Zhang Yu , Jean Philippe-Brucker , Alexander Grest Subject: [PATCH v3 1/2] iommu/arm-smmu-v3: Fix CMDQ timeout warning Date: Wed, 5 Nov 2025 11:06:37 -0800 Message-Id: <20251105190638.23172-2-jacob.pan@linux.microsoft.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251105190638.23172-1-jacob.pan@linux.microsoft.com> References: <20251105190638.23172-1-jacob.pan@linux.microsoft.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" While polling for n spaces in the cmdq, the current code instead checks if the queue is full. If the queue is almost full but not enough space ( Signed-off-by: Yu Zhang Signed-off-by: Jacob Pan --- v3: - Use a helper for cmdq poll instead of open coding (Nicolin) - Add more explanation in the commit message (Nicolin) v2: - Reduced debug print info (Nicolin) - Use a separate irq flags for exclusive lock - Handle queue_poll err code other than ETIMEOUT --- drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 43 +++++++++------------ 1 file changed, 19 insertions(+), 24 deletions(-) diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/ar= m/arm-smmu-v3/arm-smmu-v3.c index bf67d9abc901..86be84c14036 100644 --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c @@ -117,12 +117,6 @@ static bool queue_has_space(struct arm_smmu_ll_queue *= q, u32 n) return space >=3D n; } =20 -static bool queue_full(struct arm_smmu_ll_queue *q) -{ - return Q_IDX(q, q->prod) =3D=3D Q_IDX(q, q->cons) && - Q_WRP(q, q->prod) !=3D Q_WRP(q, q->cons); -} - static bool queue_empty(struct arm_smmu_ll_queue *q) { return Q_IDX(q, q->prod) =3D=3D Q_IDX(q, q->cons) && @@ -612,13 +606,13 @@ static void arm_smmu_cmdq_poll_valid_map(struct arm_s= mmu_cmdq *cmdq, __arm_smmu_cmdq_poll_set_valid_map(cmdq, sprod, eprod, false); } =20 -/* Wait for the command queue to become non-full */ -static int arm_smmu_cmdq_poll_until_not_full(struct arm_smmu_device *smmu, - struct arm_smmu_cmdq *cmdq, - struct arm_smmu_ll_queue *llq) + +static inline void arm_smmu_cmdq_poll(struct arm_smmu_device *smmu, + struct arm_smmu_cmdq *cmdq, + struct arm_smmu_ll_queue *llq, + struct arm_smmu_queue_poll *qp) { unsigned long flags; - struct arm_smmu_queue_poll qp; int ret =3D 0; =20 /* @@ -629,19 +623,19 @@ static int arm_smmu_cmdq_poll_until_not_full(struct a= rm_smmu_device *smmu, WRITE_ONCE(cmdq->q.llq.cons, readl_relaxed(cmdq->q.cons_reg)); arm_smmu_cmdq_exclusive_unlock_irqrestore(cmdq, flags); llq->val =3D READ_ONCE(cmdq->q.llq.val); - return 0; + return; } =20 - queue_poll_init(smmu, &qp); - do { - llq->val =3D READ_ONCE(cmdq->q.llq.val); - if (!queue_full(llq)) - break; - - ret =3D queue_poll(&qp); - } while (!ret); - - return ret; + ret =3D queue_poll(qp); + if (ret =3D=3D -ETIMEDOUT) { + dev_err_ratelimited(smmu->dev, "CMDQ timed out, cons: %08x, prod: 0x%08x= \n", + llq->cons, llq->prod); + /* Restart the timer */ + queue_poll_init(smmu, qp); + } else if (ret) { + dev_err_ratelimited(smmu->dev, "CMDQ poll error %d\n", ret); + } + llq->val =3D READ_ONCE(cmdq->q.llq.val); } =20 /* @@ -783,12 +777,13 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smm= u_device *smmu, local_irq_save(flags); llq.val =3D READ_ONCE(cmdq->q.llq.val); do { + struct arm_smmu_queue_poll qp; u64 old; =20 + queue_poll_init(smmu, &qp); while (!queue_has_space(&llq, n + sync)) { local_irq_restore(flags); - if (arm_smmu_cmdq_poll_until_not_full(smmu, cmdq, &llq)) - dev_err_ratelimited(smmu->dev, "CMDQ timeout\n"); + arm_smmu_cmdq_poll(smmu, cmdq, &llq, &qp); local_irq_save(flags); } =20 --=20 2.43.0 From nobody Fri Dec 19 14:23:46 2025 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id E6CA632548A for ; Wed, 5 Nov 2025 19:06:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762369605; cv=none; b=ldXs/bnZoZfrFMhIsFVLgaeL6UbhGo2O9i9/kR8iWc5uFwLlfIhn7cA/b7l23bxpbdj0UZPn5f0j1Y6r6yKMemHuZUtc6BEG8kEFiVoeu6wFUXn4mUmtnA1eQQOMjMR4ETuSj9RBCLZtzSQRprGx5HWfXluswc77rRtwjhFe0gY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762369605; c=relaxed/simple; bh=7aHJ6AEg47ScOWkAnVXL0yniYTx2hKL/5NawtqJHaoE=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=lAC2h27w9BJsYPCSdddVMfCyGKWrSP91gUHM/hEYUu0308HLcFgkyxlt3l3bF1q3ARCyCY1mIuT/d4r3bWpB7JFLSubL9NckNafBSJllyWcIY4R6j+zfqE1CMcJMy8fCwcxii1dSgeQJrEo751v+CpG6IATlgYCvO9D23nTzdzI= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=N4V+j9cT; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="N4V+j9cT" Received: from DESKTOP-0403QTC.corp.microsoft.com (unknown [20.236.11.42]) by linux.microsoft.com (Postfix) with ESMTPSA id DC7F9211D8DE; Wed, 5 Nov 2025 11:06:42 -0800 (PST) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com DC7F9211D8DE DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1762369603; bh=nFPlGNBmuoTHyaq4WoUsv2XfpHj3tlsB+4yFRJpUEr4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=N4V+j9cTpSUm1iWQ1YvzNfELnp6cD+T2J75u5o5VAuS+TTzbhirIxqS7oXqDQ8i1Z oiMTyTn4WDGpm+bkVyFIt+cgmRTStAs/w4HwbUZ6g5M7sM4dwCPuaKRmv0VYHPqDX9 Zb49QxY/+eR+y6DNCDivDG0w3OcSr2DWbwMg9HYw= From: Jacob Pan To: linux-kernel@vger.kernel.org, "iommu@lists.linux.dev" , Will Deacon , Joerg Roedel , Mostafa Saleh , Jason Gunthorpe , Robin Murphy , Nicolin Chen Cc: Jacob Pan , Zhang Yu , Jean Philippe-Brucker , Alexander Grest Subject: [PATCH v3 2/2] iommu/arm-smmu-v3: Improve CMDQ lock fairness and efficiency Date: Wed, 5 Nov 2025 11:06:38 -0800 Message-Id: <20251105190638.23172-3-jacob.pan@linux.microsoft.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251105190638.23172-1-jacob.pan@linux.microsoft.com> References: <20251105190638.23172-1-jacob.pan@linux.microsoft.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Alexander Grest The SMMU CMDQ lock is highly contentious when there are multiple CPUs issuing commands and the queue is nearly full. The lock has the following states: - 0: Unlocked - >0: Shared lock held with count - INT_MIN+N: Exclusive lock held, where N is the # of shared waiters - INT_MIN: Exclusive lock held, no shared waiters When multiple CPUs are polling for space in the queue, they attempt to grab the exclusive lock to update the cons pointer from the hardware. If they fail to get the lock, they will spin until either the cons pointer is updated by another CPU. The current code allows the possibility of shared lock starvation if there is a constant stream of CPUs trying to grab the exclusive lock. This leads to severe latency issues and soft lockups. Consider the following scenario where CPU1's attempt to acquire the shared lock is starved by CPU2 and CPU0 contending for the exclusive lock. CPU0 (exclusive) | CPU1 (shared) | CPU2 (exclusive) | `cmdq->lock` Reviewed-by: Mostafa Saleh Reviewed-by: Nicolin Chen -------------------------------------------------------------------------- trylock() //takes | | | 0 | shared_lock() | | INT_MIN | fetch_inc() | | INT_MIN | no return | | INT_MIN + 1 | spins // VAL >=3D 0 | | INT_MIN + 1 unlock() | spins... | | INT_MIN + 1 set_release(0) | spins... | | 0 see[NOTE] (done) | (sees 0) | trylock() // takes | 0 | *exits loop* | cmpxchg(0, INT_MIN) | 0 | | *cuts in* | INT_MIN | cmpxchg(0, 1) | | INT_MIN | fails // !=3D 0 | | INT_MIN | spins // VAL >=3D 0 | | INT_MIN | *starved* | | INT_MIN [NOTE] The current code resets the exclusive lock to 0 regardless of the state of the lock. This causes two problems: 1. It opens the possibility of back-to-back exclusive locks and the downstream effect of starving shared lock. 2. The count of shared lock waiters are lost. To mitigate this, we release the exclusive lock by only clearing the sign bit while retaining the shared lock waiter count as a way to avoid starving the shared lock waiters. Also deleted cmpxchg loop while trying to acquire the shared lock as it is not needed. The waiters can see the positive lock count and proceed immediately after the exclusive lock is released. Exclusive lock is not starved in that submitters will try exclusive lock first when new spaces become available. Reviewed-by: Mostafa Saleh Reviewed-by: Nicolin Chen Signed-off-by: Alexander Grest Signed-off-by: Jacob Pan --- drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 31 ++++++++++++++------- 1 file changed, 21 insertions(+), 10 deletions(-) diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/ar= m/arm-smmu-v3/arm-smmu-v3.c index aa7d4696b351..b5ab2f330edf 100644 --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c @@ -481,20 +481,26 @@ static void arm_smmu_cmdq_skip_err(struct arm_smmu_de= vice *smmu) */ static void arm_smmu_cmdq_shared_lock(struct arm_smmu_cmdq *cmdq) { - int val; - /* - * We can try to avoid the cmpxchg() loop by simply incrementing the - * lock counter. When held in exclusive state, the lock counter is set - * to INT_MIN so these increments won't hurt as the value will remain - * negative. + * When held in exclusive state, the lock counter is set to INT_MIN + * so these increments won't hurt as the value will remain negative. + * The increment will also signal the exclusive locker that there are + * shared waiters. */ if (atomic_fetch_inc_relaxed(&cmdq->lock) >=3D 0) return; =20 - do { - val =3D atomic_cond_read_relaxed(&cmdq->lock, VAL >=3D 0); - } while (atomic_cmpxchg_relaxed(&cmdq->lock, val, val + 1) !=3D val); + /* + * Someone else is holding the lock in exclusive state, so wait + * for them to finish. Since we already incremented the lock counter, + * no exclusive lock can be acquired until we finish. We don't need + * the return value since we only care that the exclusive lock is + * released (i.e. the lock counter is non-negative). + * Once the exclusive locker releases the lock, the sign bit will + * be cleared and our increment will make the lock counter positive, + * allowing us to proceed. + */ + atomic_cond_read_relaxed(&cmdq->lock, VAL > 0); } =20 static void arm_smmu_cmdq_shared_unlock(struct arm_smmu_cmdq *cmdq) @@ -521,9 +527,14 @@ static bool arm_smmu_cmdq_shared_tryunlock(struct arm_= smmu_cmdq *cmdq) __ret; \ }) =20 +/* + * Only clear the sign bit when releasing the exclusive lock this will + * allow any shared_lock() waiters to proceed without the possibility + * of entering the exclusive lock in a tight loop. + */ #define arm_smmu_cmdq_exclusive_unlock_irqrestore(cmdq, flags) \ ({ \ - atomic_set_release(&cmdq->lock, 0); \ + atomic_fetch_and_release(~INT_MIN, &cmdq->lock); \ local_irq_restore(flags); \ }) =20 --=20 2.43.0