RE: [PATCH] x86/resctrl: Fix mbm_setup_overflow_handler() when last CPU goes offline

[Luck, Tony]

> 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@xxxxxxxxx>
Reviewed-by: Tont Luck <tony.luck@xxxxxxxxx>

-Tony