Re: Backport request: Revert "softirq: Let ksoftirqd do its job" (d15121be7485)

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Oct 30, 2026 at 02:16:57PM +0800, Jason Xing wrote:
> Hi Paolo,
> 
> On Thu, Oct 29, 2026 at 4:33 PM Paolo Abeni <pabeni@redhat.com> wrote:
> >
> > On 10/29/26 06:42, Gyutae Bae wrote:
> > > Please backport the following upstream commit to 6.1.y and 5.15.y:
> > >
> > >    d15121be7485655129101f3960ae6add40204463
> > >    Revert "softirq: Let ksoftirqd do its job"
> > >
> > > On a production 5.15 PREEMPT_NONE node, long-running overlayfs
> > > getdents64() calls prevented ksoftirqd from running.  RPS IPIs
> > > continued to arrive, but NET_RX processing on that CPU stalled for
> > > up to about 3 seconds.  Over 15 days, the node logged about 1,000
> > > bonding ARP monitor failovers, with no link-state changes recorded
> > > during that period.  The failovers stopped after the workload was
> > > removed.
> > >
> > > On a 5.15.27 PREEMPT_NONE test node, we used a livepatch to make
> > > ksoftirqd_running() always return false.  This removes the same
> > > softirq deferral checks as the revert on this build.  Under the same
> > > load, we measured three 10-minute windows: before patching, with the
> > > livepatch active, and after removing it.  The number of HTTP requests
> > > taking more than 200 ms was 17, 0 and 11, respectively.  TCPTimeouts
> > > increased by 25, 1 and 21.
> > >
> > > We verified with patch --dry-run that the revert applies cleanly
> > > to v6.1.190 and v5.15.223.  Both already contain the related RCU
> > > fix 1dd1eff161bd.  6.6.y and later already contain the revert.
> > >
> > > The original commit was intended to prevent user-task starvation
> > > under a softirq flood.  The revert's changelog points to threaded
> > > NAPI as an alternative, but it is disabled by default.
> > >
> > > The revert was merged in v6.5 without a Cc: stable tag.
> > > Syzbot has also reported bisections to this revert [1].
> > > Paolo, Thomas, Eric: I would appreciate your thoughts on whether
> > > this revert is suitable for 6.1.y and 5.15.y.
> > >
> > > [1] https://lore.kernel.org/all/000000000000a249880603717c15@google.com/
> > IMHO such change should not land to stable lightly. It's a behavioural
> > change with subtle side effects. The threaded napi feature is disabled
> 
> Technically speaking, yes, it's actually not a bug but an optimization.
> 
> > by default for the same reason. Also the revert has some soft functional
> > dependencies in napi threaded cleanup and improvements landed into v6.4,
> > i.e. 6bb218b5a6e3f3c5e2e76faf8fc976f3f6f72496, likely more.
> 
> Maybe I have a slightly different thought on this: since I really
> received/noticed there are not a few reports showing the 'issue' in
> the older kernels. It's not easy for some people or company to upgrade
> the stable kernel. I'm wondering if it's still possible to land it
> with more careful consideration in the patch if Gyutae volunteers to
> do so? Please feel free to reject the proposal :)

Given that 5.15.y is about to go end-of-life in a month, that's a
non-starter.

For 6.1.y, you better be planning how to move off of that _now_ given
that you only have 1 more year before that is gone.  Anyone hitting this
issue should be moving now as that gives you an incentive to do so :)

thanks,

greg k-h