linux-kernel - [RFC] Is lib/irq_poll still considered useful?

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Date:   Thu, 30 Sep 2021 12:56:05 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>, linux-rdma@...r.kernel.org,
        Subbu Seetharaman <subbu.seetharaman@...adcom.com>,
        Ketan Mukadam <ketan.mukadam@...adcom.com>,
        Jitendra Bhivare <jitendra.bhivare@...adcom.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "Manoj N. Kumar" <manoj@...ux.ibm.com>,
        "Matthew R. Ochs" <mrochs@...ux.ibm.com>,
        Uma Krishnan <ukrishn@...ux.ibm.com>,
        Brian King <brking@...ibm.com>,
        James Smart <james.smart@...adcom.com>,
        Dick Kennedy <dick.kennedy@...adcom.com>,
        Kashyap Desai <kashyap.desai@...adcom.com>,
        Sumit Saxena <sumit.saxena@...adcom.com>,
        Shivasharan S <shivasharan.srikanteshwara@...adcom.com>,
        megaraidlinux.pdl@...adcom.com,
        Sathya Prakash <sathya.prakash@...adcom.com>,
        Sreekanth Reddy <sreekanth.reddy@...adcom.com>,
        Suganath Prabu Subramani 
        <suganath-prabu.subramani@...adcom.com>,
        MPT-FusionLinux.pdl@...adcom.com
Subject: [RFC] Is lib/irq_poll still considered useful?

I was looking at irq_poll and for missing scheduling points.
It raised the question why are there 7 driver still using irq_poll and
not moved on to something else like threaded interrupts or kworker.

There is:
- Infiband can complete direct, irq_poll and kworker.
- be2iscsi only irq_poll.
- cxlflash only irq_poll. Not sure how IRQs are acked.
- ipr direct or irq_poll, can be configured. Now sure how IRQs are acked.
- lpfc kworker and/or irq_poll. Not sure all invocations are from
  interrupts like context [0].
- megaraid irq_poll. Not sure all invocations are from interrupts like
  context [0].
- mpt3sas irq_poll or io_uring io poll. Not sure all invocations are
  from interrupts like context [0].

[0] If irq_poll_sched() is not used from an interrupt (as in interrupt
service routine, timer handler, tasklet (not encouraging just noticed))
but from task context (as in kworker for instance) then irq-poll handler
will not be invoked right away. Instead it will be delayed to random
point in time until an interrupt fires or something down the stack
performs a pending softirq check.
Waking ksoftirqd itself isn't helping much since it will set a
NEED_RESCHED bit in the task_struct which local_irq_restore() isn't
testing for. So the scheduling event is delayed until spin_unlock() for
instance.

Is there a reason for the remaining user of irq_poll to keep using it?

Sebastian