On Sun, Apr 18, 2021 at 05:54:26PM -0400, Paul Gortmaker wrote:
> After commit 3662daf02350 ("sched/isolation: Allow "isolcpus=" to skip
> unknown sub-parameters") the isolcpus= string is walked to skip over what
> might be any future flag comma separated additions.
>
> However, there is a logic error, and so as can clearly be seen below, it
> will ignore its own arg len and search to the end of the bootarg string.
>
> $ dmesg|grep isol
> Command line: BOOT_IMAGE=/boot/bzImage isolcpus=xyz pleasedontparseme=1
> root=/dev/sda1 ro
> isolcpus: Skipped unknown flag xyz
> isolcpus: Invalid flag pleasedontparseme=1 root=/dev/sda1 ro
>
> This happens because the flag "skip" code does an unconditional
> increment, which skips over the '\0' check the loop body looks for. If
> the isolcpus= happens to be the last bootarg, then you'd never notice?
>
> So we only increment if the skipped flag is followed by a comma, as per
> what the existing "continue" flag matching code does.
>
> Note that isolcpus= was declared deprecated as of v4.15 (b0d40d2b22fe),
> so we might want to revisit that if we are trying to future-proof it
> as recently as a year ago for as yet unseen new flags.
Thanks for report the issue.
Is cpuset going to totally replace "isolcpus="? It seems most hk_flags will be
handled by nohz_full=, and HK_FLAG_DOMAIN can be done by cpuset. However it
seems still the only place to set the new flag HK_FLAG_MANAGED_IRQ. If one day
we'll finally obsolete isolcpus= we may need to think about where to put it?
When I looked at it, I also noticed I see no caller to set HK_FLAG_SCHED at
all. Is it really used anywhere?
Regarding this patch...
>
> Cc: Peter Xu <pet...@redhat.com>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Frederic Weisbecker <frede...@kernel.org>
> Fixes: 3662daf02350 ("sched/isolation: Allow "isolcpus=" to skip unknown
> sub-parameters")
> Signed-off-by: Paul Gortmaker <paul.gortma...@windriver.com>
>
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 5a6ea03f9882..9652dba7e938 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -188,7 +188,8 @@ static int __init housekeeping_isolcpus_setup(char *str)
> }
>
> pr_info("isolcpus: Skipped unknown flag %.*s\n", len, par);
> - str++;
> + if (str[1] == ',') /* above continue; match on "flag," */
.. wondering why it is not "str[0] == ','" instead?
Thanks,
> + str++;
> }
>
> /* Default behaviour for isolcpus without flags */
> --
> 2.25.1
>
--
Peter Xu
