linux-kernel - Re: [patch 27/30] xen/events: Only force affinity mask for percpu interrupts

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Message-ID: <2164a0ce-0e0d-c7dc-ac97-87c8f384ad82@suse.com>
Date:   Fri, 11 Dec 2020 13:10:43 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     boris.ostrovsky@...cle.com, Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Marc Zyngier <maz@...nel.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        afzal mohammed <afzal.mohd.ma@...il.com>,
        linux-parisc@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, linux-s390@...r.kernel.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Wambui Karuga <wambui.karugax@...il.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, Lee Jones <lee.jones@...aro.org>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Michal Simek <michal.simek@...inx.com>,
        linux-pci@...r.kernel.org,
        Karthikeyan Mitran <m.karthikeyan@...iveil.co.in>,
        Hou Zhiqiang <Zhiqiang.Hou@....com>,
        Tariq Toukan <tariqt@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [patch 27/30] xen/events: Only force affinity mask for percpu
 interrupts

On 11.12.20 00:20, boris.ostrovsky@...cle.com wrote:
> 
> On 12/10/20 2:26 PM, Thomas Gleixner wrote:
>> All event channel setups bind the interrupt on CPU0 or the target CPU for
>> percpu interrupts and overwrite the affinity mask with the corresponding
>> cpumask. That does not make sense.
>>
>> The XEN implementation of irqchip::irq_set_affinity() already picks a
>> single target CPU out of the affinity mask and the actual target is stored
>> in the effective CPU mask, so destroying the user chosen affinity mask
>> which might contain more than one CPU is wrong.
>>
>> Change the implementation so that the channel is bound to CPU0 at the XEN
>> level and leave the affinity mask alone. At startup of the interrupt
>> affinity will be assigned out of the affinity mask and the XEN binding will
>> be updated.
> 
> 
> If that's the case then I wonder whether we need this call at all and instead bind at startup time.

After some discussion with Thomas on IRC and xen-devel archaeology the
result is: this will be needed especially for systems running on a
single vcpu (e.g. small guests), as the .irq_set_affinity() callback
won't be called in this case when starting the irq.


Juergen

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