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[47.54.130.67]) by smtp.gmail.com with ESMTPSA id af79cd13be357-92e80040ff1sm81033885a.21.2026.07.01.17.16.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Jul 2026 17:16:04 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1wf563-00000004cjY-2WuA; Wed, 01 Jul 2026 21:16:03 -0300 Date: Wed, 1 Jul 2026 21:16:03 -0300 From: Jason Gunthorpe To: "Kiryl Shutsemau (Meta)" Cc: Will Deacon , Robin Murphy , Joerg Roedel , Nicolin Chen , Kyle McMartin , Breno Leitao , Usama Arif , linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH] iommu/arm-smmu-v3: Shrink command/event/PRI queues in kdump kernel Message-ID: <20260702001603.GN7525@ziepe.ca> References: <20260701154528.768976-1-kas@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260701154528.768976-1-kas@kernel.org> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260701_171607_095039_96D52542 X-CRM114-Status: GOOD ( 18.63 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Jul 01, 2026 at 04:45:28PM +0100, Kiryl Shutsemau (Meta) wrote: > The command, event and PRI queues are sized from the maxima the hardware > advertises in IDR1, which can be several megabytes each. On systems with > many SMMUv3 instances that cost is paid per instance and adds up to tens > of megabytes of coherent DMA in the capture kernel. > > A kdump capture kernel runs from a small crashkernel reservation and only > has to drive the few devices used to save the dump, so deep queues serve > no purpose. The queues carry invalidation commands and fault records, not > DMA data, so dump throughput is unaffected; a shallower queue only bounds > how many commands may be in flight before a sync, which does not matter for > the capture kernel's small device count and modest I/O. > > Clamp every queue to a single page when is_kdump_kernel() is true. Doing > it in arm_smmu_init_one_queue() covers the command, event and PRI queues > in one place. The command queue still holds at least one batch plus a sync > (256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so > command batching keeps working. > > Suggested-by: Kyle McMartin > Signed-off-by: Kiryl Shutsemau (Meta) > --- > drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) Make sense to me > + if (is_kdump_kernel()) { > + u32 ent_sz_shift = ilog2(dwords) + 3; > + > + q->llq.max_n_shift = min_t(u32, q->llq.max_n_shift, > + PAGE_SHIFT - ent_sz_shift); I saw lately many people saying you should not use min_t, why is it needed here? Jason