linux-kernel - RE: [PATCH] x86/resctrl: Fix mbm_setup_overflow

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Date: Wed, 27 Mar 2024 23:01:10 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Chatre, Reinette" <reinette.chatre@...el.com>, Borislav Petkov
	<bp@...en8.de>
CC: "Yu, Fenghua" <fenghua.yu@...el.com>, "Wieczor-Retman, Maciej"
	<maciej.wieczor-retman@...el.com>, Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>, "Drew
 Fustini" <dfustini@...libre.com>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: RE: [PATCH] x86/resctrl: Fix mbm_setup_overflow_handler() when last
 CPU goes offline

> There seems to be two issues here (although I am not familiar with these flows). First,
> it seems that tick_nohz_full_mask is not actually allocated unless the user boots
> with a "nohz_full=". This means that any attempt to access bits within tick_nohz_full_mask
> will cause this OOPS. If that is allocated then the second issue seems that the  
> buried __ffs() call requires that it not be called with 0 and this checking is not done.
>
> To me it seems most appropriate to fix this at the central place to ensure all scenarios
> are handled instead of scattering checks.

Good analysis.

> To that end, what do you think of something like below? It uses tick_nohz_full_enabled() check
> to ensure that tick_nohz_full_mask is actually allocated while the other changes aim to
> avoid __ffs() on 0.

Looks good.

Tested-by: Tony Luck <tony.luck@...el.com>
Reviewed-by: Tont Luck <tony.luck@...el.com>

-Tony