linux-kernel - [PATCH v5 9/9] doc: update managed

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Message-Id: <20250110-isolcpus-io-queues-v5-9-0e4f118680b0@kernel.org>
Date: Fri, 10 Jan 2025 17:26:47 +0100
From: Daniel Wagner <wagi@...nel.org>
To: Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>, 
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>, 
 "Michael S. Tsirkin" <mst@...hat.com>
Cc: "Martin K. Petersen" <martin.petersen@...cle.com>, 
 Thomas Gleixner <tglx@...utronix.de>, 
 Costa Shulyupin <costa.shul@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
 Valentin Schneider <vschneid@...hat.com>, Waiman Long <llong@...hat.com>, 
 Ming Lei <ming.lei@...hat.com>, Frederic Weisbecker <frederic@...nel.org>, 
 Mel Gorman <mgorman@...e.de>, Hannes Reinecke <hare@...e.de>, 
 linux-kernel@...r.kernel.org, linux-block@...r.kernel.org, 
 linux-nvme@...ts.infradead.org, megaraidlinux.pdl@...adcom.com, 
 linux-scsi@...r.kernel.org, storagedev@...rochip.com, 
 virtualization@...ts.linux.dev, GR-QLogic-Storage-Upstream@...vell.com, 
 Daniel Wagner <wagi@...nel.org>
Subject: [PATCH v5 9/9] doc: update managed_irq documentation

The managed_irq documentation is a bit difficult to understand. Rephrase
the current text and add the latest changes how managed_irq CPU sets are
handled.

Isolated CPUs and housekeeping CPUs are grouped into sets and the
possibility of stalls if all housekeeping CPUs are offlined in a set.

Signed-off-by: Daniel Wagner <wagi@...nel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 46 +++++++++++++------------
 1 file changed, 24 insertions(+), 22 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3872bc6ec49d63772755504966ae70113f24a1db..e4bf1fc984943c1d4938dffb85d97da05010a325 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2460,28 +2460,30 @@
 			  "number of CPUs in system - 1".

 			managed_irq
-
-			  Isolate from being targeted by managed interrupts
-			  which have an interrupt mask containing isolated
-			  CPUs. The affinity of managed interrupts is
-			  handled by the kernel and cannot be changed via
-			  the /proc/irq/* interfaces.
-
-			  This isolation is best effort and only effective
-			  if the automatically assigned interrupt mask of a
-			  device queue contains isolated and housekeeping
-			  CPUs. If housekeeping CPUs are online then such
-			  interrupts are directed to the housekeeping CPU
-			  so that IO submitted on the housekeeping CPU
-			  cannot disturb the isolated CPU.
-
-			  If a queue's affinity mask contains only isolated
-			  CPUs then this parameter has no effect on the
-			  interrupt routing decision, though interrupts are
-			  only delivered when tasks running on those
-			  isolated CPUs submit IO. IO submitted on
-			  housekeeping CPUs has no influence on those
-			  queues.
+			  Isolate CPUs from IRQ-related work for drivers
+			  that support managed interrupts, ensuring no
+			  IRQ work is scheduled on the isolated CPUs. The
+			  kernel manages the affinity of managed
+			  interrupts, which cannot be changed via the
+			  /proc/irq/* interfaces.
+
+			  Since isolated CPUs do not handle IRQ work, the
+			  work is forwarded to housekeeping CPUs.
+			  Housekeeping and isolated CPUs are grouped into
+			  sets, ensuring at least one housekeeping CPU is
+			  available per set. Consequently, if all
+			  housekeeping CPUs in a set are offlined, there
+			  will be no CPU available to handle IRQ work for
+			  the isolated CPUs. Therefore, users should
+			  offline all isolated CPUs before offlining the
+			  housekeeping CPUs in a set to avoid stalls.
+
+			  The block layer ensures that no I/O is
+			  scheduled on isolated CPU, except when user
+			  applications running on the isolated CPUs issue
+			  I/O requests. In this case the I/O is issued
+			  from the isolated CPU and the IRQ related work
+			  is forwared to a housekeeping CPU.

 			The format of <cpu-list> is described above.

-- 
2.47.1