Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se

[Biju Das]

Subject: Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se
Date: Wed, 4 Oct 2023 22:39:39 +0200	[thread overview]
Message-ID: <c92bc8a6-225d-4fd2-88b5-8994090fb2de@xxxxxxxxxxx> (raw)
In-Reply-To: <xm261qego72d.fsf_-_@xxxxxxxxxx>

Hi,

On 30.09.2023 02:09, Benjamin Segall wrote:
> The old pick_eevdf could fail to find the actual earliest eligible
> deadline when it descended to the right looking for min_deadline, but it
> turned out that that min_deadline wasn't actually eligible. In that case
> we need to go back and search through any left branches we skipped
> looking for the actual best _eligible_ min_deadline.
>
> This is more expensive, but still O(log n), and at worst should only
> involve descending two branches of the rbtree.
>
> I've run this through a userspace stress test (thank you
> tools/lib/rbtree.c), so hopefully this implementation doesn't miss any
> corner cases.
>
> Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
> Signed-off-by: Ben Segall <bsegall@xxxxxxxxxx>

This patch causing issues [1] in Renesas RZ/G2L SMARC EVK platform. Reverting the patch fixes the warning messages

[1]
[   25.550898] EEVDF scheduling fail, picking leftmost

[   15.109634] ======================================================
[   15.109636] WARNING: possible circular locking dependency detected
[   15.109641] 6.6.0-rc4-next-20231005-arm64-renesas-ga03f9ebbbb4c #1165 Not tainted
[   15.109648] ------------------------------------------------------
[   15.109649] migration/0/16 is trying to acquire lock:
[   15.109654] ffff800081713460 (console_owner){..-.}-{0:0}, at: console_flush_all.constprop.0+0x1a0/0x438
[   15.109694]
[   15.109694] but task is already holding lock:
[   15.109697] ffff00007fbd2298 (&rq->__lock){-.-.}-{2:2}, at: __schedule+0xd0/0xbe0
[   15.109718]
[   15.109718] which lock already depends on the new lock.
[   15.109718]
[   15.109720]
[   15.109720] the existing dependency chain (in reverse order) is:

   25.551560]  __down_trylock_console_sem+0x34/0xb8
[   25.551567]  console_trylock+0x24/0x74
[   25.551574]  vprintk_emit+0x114/0x388
[   25.551581]  vprintk_default+0x34/0x3c
[   25.551588]  vprintk+0x9c/0xb4
[   25.551594]  _printk+0x58/0x7c
[   25.551600]  pick_next_task_fair+0x274/0x480
[   25.551608]  __schedule+0x154/0xbe0
[   25.551616]  schedule+0x48/0x110
[   25.551623]  worker_thread+0x1b8/0x3f8
[   25.551630]  kthread+0x114/0x118
[   25.551635]  ret_from_fork+0x10/0x20
[  OK  ] Started System Logging Service.
[   26.099203] EEVDF scheduling fail, picking leftmost

Cheers,
Biju