linux-kernel - Re: [PATCH mips-fixes] MIPS: smp: fill in sibling and core maps earlier

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Date:   Mon, 14 Feb 2022 20:00:12 +0100
From:   Philippe Mathieu-Daudé <f4bug@...at.org>
To:     Alexander Lobakin <alobakin@...me>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mips-fixes] MIPS: smp: fill in sibling and core maps
 earlier

On 12/2/22 23:21, Alexander Lobakin wrote:
> After enabling CONFIG_SCHED_CORE (landed during 5.14 cycle),
> 2-core 2-thread-per-core interAptiv (CPS-driven) started emitting
> the following:
> 
> [    0.025698] CPU1 revision is: 0001a120 (MIPS interAptiv (multi))
> [    0.048183] ------------[ cut here ]------------
> [    0.048187] WARNING: CPU: 1 PID: 0 at kernel/sched/core.c:6025 sched_core_cpu_starting+0x198/0x240
> [    0.048220] Modules linked in:
> [    0.048233] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.17.0-rc3+ #35 b7b319f24073fd9a3c2aa7ad15fb7993eec0b26f
> [    0.048247] Stack : 817f0000 00000004 327804c8 810eb050 00000000 00000004 00000000 c314fdd1
> [    0.048278]         830cbd64 819c0000 81800000 817f0000 83070bf4 00000001 830cbd08 00000000
> [    0.048307]         00000000 00000000 815fcbc4 00000000 00000000 00000000 00000000 00000000
> [    0.048334]         00000000 00000000 00000000 00000000 817f0000 00000000 00000000 817f6f34
> [    0.048361]         817f0000 818a3c00 817f0000 00000004 00000000 00000000 4dc33260 0018c933
> [    0.048389]         ...
> [    0.048396] Call Trace:
> [    0.048399] [<8105a7bc>] show_stack+0x3c/0x140
> [    0.048424] [<8131c2a0>] dump_stack_lvl+0x60/0x80
> [    0.048440] [<8108b5c0>] __warn+0xc0/0xf4
> [    0.048454] [<8108b658>] warn_slowpath_fmt+0x64/0x10c
> [    0.048467] [<810bd418>] sched_core_cpu_starting+0x198/0x240
> [    0.048483] [<810c6514>] sched_cpu_starting+0x14/0x80
> [    0.048497] [<8108c0f8>] cpuhp_invoke_callback_range+0x78/0x140
> [    0.048510] [<8108d914>] notify_cpu_starting+0x94/0x140
> [    0.048523] [<8106593c>] start_secondary+0xbc/0x280
> [    0.048539]
> [    0.048543] ---[ end trace 0000000000000000 ]---
> [    0.048636] Synchronize counters for CPU 1: done.
> 
> ...for each but CPU 0/boot.
> Basic debug printks right before the mentioned line say:
> 
> [    0.048170] CPU: 1, smt_mask:
> 
> So smt_mask, which is sibling mask obviously, is empty when entering
> the function.
> This is critical, as sched_core_cpu_starting() calculates
> core-scheduling parameters only once per CPU start, and it's crucial
> to have all the parameters filled in at that moment (at least it
> uses cpu_smt_mask() which in fact is `&cpu_sibling_map[cpu]` on
> MIPS).
> 
> A bit of debugging led me to that set_cpu_sibling_map() performing
> the actual map calculation, was being invocated after
> notify_cpu_start(), and exactly the latter function starts CPU HP
> callback round (sched_core_cpu_starting() is basically a CPU HP
> callback).
> While the flow is same on ARM64 (maps after the notifier, although
> before calling set_cpu_online()), x86 started calculating sibling
> maps earlier than starting the CPU HP callbacks in Linux 4.14 (see
> [0] for the reference). Neither me nor my brief tests couldn't find
> any potential caveats in calculating the maps right after performing
> delay calibration, but the WARN splat is now gone.
> The very same debug prints now yield exactly what I expected from
> them:
> 
> [    0.048433] CPU: 1, smt_mask: 0-1
> 
> [0] https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=76ce7cfe35ef

Isn't it worth Cc'ing stable@...r.kernel.org here?

> Signed-off-by: Alexander Lobakin <alobakin@...me>
> ---
>   arch/mips/kernel/smp.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <f4bug@...at.org>