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Date:   Thu, 4 Feb 2021 15:15:46 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Nitesh Narayan Lal <nitesh@redhat.com>,
        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,
        bhelgaas@google.com, linux-pci@vger.kernel.org,
        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: <20210204181546.GA30113@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>
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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.

How about adding a new flag for isolcpus instead?