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Date:   Thu, 4 Feb 2021 16:06:47 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Nitesh Narayan Lal <nitesh@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Robin Murphy <robin.murphy@arm.com>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        frederic@kernel.org, juri.lelli@redhat.com, abelits@marvell.com,
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        rostedt@goodmis.org, mingo@kernel.org, peterz@infradead.org,
        davem@davemloft.net, akpm@linux-foundation.org,
        sfr@canb.auug.org.au, stephen@networkplumber.org,
        rppt@linux.vnet.ibm.com, jinyuqi@huawei.com,
        zhangshaokun@hisilicon.com
Subject: Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping
 CPUs
Message-ID: <20210204190647.GA32868@fuller.cnet>
References: <20200625223443.2684-1-nitesh@redhat.com>
 <20200625223443.2684-2-nitesh@redhat.com>
 <3e9ce666-c9cd-391b-52b6-3471fe2be2e6@arm.com>
 <20210127121939.GA54725@fuller.cnet>
 <87r1m5can2.fsf@nanos.tec.linutronix.de>
 <20210128165903.GB38339@fuller.cnet>
 <87h7n0de5a.fsf@nanos.tec.linutronix.de>
 <20210204181546.GA30113@fuller.cnet>
 <cfa138e9-38e3-e566-8903-1d64024c917b@redhat.com>
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On Thu, Feb 04, 2021 at 01:47:38PM -0500, Nitesh Narayan Lal wrote:
> 
> On 2/4/21 1:15 PM, Marcelo Tosatti wrote:
> > On Thu, Jan 28, 2021 at 09:01:37PM +0100, Thomas Gleixner wrote:
> >> On Thu, Jan 28 2021 at 13:59, Marcelo Tosatti wrote:
> >>>> The whole pile wants to be reverted. It's simply broken in several ways.
> >>> I was asking for your comments on interaction with CPU hotplug :-)
> >> Which I answered in an seperate mail :)
> >>
> >>> So housekeeping_cpumask has multiple meanings. In this case:
> >> ...
> >>
> >>> So as long as the meaning of the flags are respected, seems
> >>> alright.
> >> Yes. Stuff like the managed interrupts preference for housekeeping CPUs
> >> when a affinity mask spawns housekeeping and isolated is perfectly
> >> fine. It's well thought out and has no limitations.
> >>
> >>> Nitesh, is there anything preventing this from being fixed
> >>> in userspace ? (as Thomas suggested previously).
> >> Everything with is not managed can be steered by user space.
> > Yes, but it seems to be racy (that is, there is a window where the 
> > interrupt can be delivered to an isolated CPU).
> >
> > ethtool ->
> > xgbe_set_channels ->
> > xgbe_full_restart_dev ->
> > xgbe_alloc_memory ->
> > xgbe_alloc_channels ->
> > cpumask_local_spread
> >
> > Also ifconfig eth0 down / ifconfig eth0 up leads
> > to cpumask_spread_local.
> 
> There's always that possibility.

Then there is a window where isolation can be broken.

> We have to ensure that we move the IRQs by a tuned daemon or some other
> userspace script every time there is a net-dev change (eg. device comes up,
> creates VFs, etc).

Again, race window open can result in interrupt to isolated CPU.

> > How about adding a new flag for isolcpus instead?
> >
> 
> Do you mean a flag based on which we can switch the affinity mask to
> housekeeping for all the devices at the time of IRQ distribution?

Yes a new flag for isolcpus. HK_FLAG_IRQ_SPREAD or some better name.