Re: [PATCH 09/12] x86/mm: enable broadcast TLB invalidation for multi-threaded processes

[Rik van Riel <riel@surriel.com>]

On Wed, 2025-01-01 at 16:15 +0000, Karim Manaouil wrote:
> On Wed, Jan 01, 2025 at 05:20:01PM +0200, Nadav Amit wrote:
> > 
> > 
> > On 01/01/2025 6:42, Rik van Riel wrote:
> > > Fixed for the next version.
> > 
> > Thanks Rik,
> > 
> > Admittedly, I don't feel great about my overall last review - it
> > mostly
> > focused on style and common BKMs.
> > 
> > I still don't quite get the entire logic. To name one thing that I
> > don't
> > understand: why do we need broadcast_asid_list and the complicated
> > games of
> > syncing it with broadcast_asid_used. Why wouldn't
> > broadcast_asid_used
> > suffice?
> 
> If I uderstand correctly from Rik's patch, I think the list is needed
> to
> save the flush for only when we run out of the ASID space (wrap
> around).
> Without the list, whenever the ASID bit is cleared, you also have to
> flush
> the TLBs.

That's exactly it.

The list will only contain processes that are active on
multiple CPUs, and hit a TLB flush "at the right moment"
to be assigned a broadcast ASID, which will be true for
essentially every process that does a lot of TLB flushes
and is long lived.

However, something like a kernel build has lots of
short lived, single threaded processes, for which we
should not be using broadcast TLB flushing, and which
will not need to remove themselves from the list at
exit time.

-- 
All Rights Reversed.