Le Wed, Oct 29, 2025 at 03:13:59PM +0100, Valentin Schneider a écrit :

On 29/10/25 11:31, Frederic Weisbecker wrote:

quoted

Le Wed, Oct 29, 2025 at 11:16:23AM +0100, Valentin Schneider a écrit :

quoted

On 28/10/25 16:59, Frederic Weisbecker wrote:

quoted

Le Fri, Oct 10, 2025 at 05:38:37PM +0200, Valentin Schneider a écrit :

quoted

@@ -171,8 +172,27 @@ For 32-bit we have the following conventions - kernel is built with
     andq    $(~PTI_USER_PGTABLE_AND_PCID_MASK), \reg
 .endm

-.macro COALESCE_TLBI
+.macro COALESCE_TLBI scratch_reg:req
 #ifdef CONFIG_COALESCE_TLBI
+	/* No point in doing this for housekeeping CPUs */
+	movslq  PER_CPU_VAR(cpu_number), \scratch_reg
+	bt	\scratch_reg, tick_nohz_full_mask(%rip)
+	jnc	.Lend_tlbi_\@

I assume it's not possible to have a static call/branch to
take care of all this ?

I think technically yes, but that would have to be a per-cpu patchable
location, which would mean something like each CPU having its own copy of
that text page... Unless there's some existing way to statically optimize

  if (cpumask_test_cpu(smp_processor_id(), mask))

where @mask is a boot-time constant (i.e. the nohz_full mask).

Or just check housekeeping_overriden static key before everything. This one is
enabled only if either nohz_full, isolcpus or cpuset isolated partition (well,
it's on the way for the last one) are running, but those are all niche, which
means you spare 99.999% kernel usecases.

Oh right, if NOHZ_FULL is actually in use.

Yeah that housekeeping key could do since, at least for the cmdline
approach, it's set during start_kernel(). I need to have a think about the
runtime cpuset case.

You can ignore the runtime thing and simply check the static key before reading
the housekeeping mask. For now nohz_full is only enabled by cmdline.

Given we have ALTERNATIVE's in there I assume something like a
boot-time-driven static key could do, but I haven't found out yet if and
how that can be shoved in an ASM file.

Right, I thought I had seen static keys in ASM already but I can't find it
anymore. arch/x86/include/asm/jump_label.h is full of reusable magic
though.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs