Re: [PATCH 0/4] Introduce QPW for per-cpu operations

[Vlastimil Babka <vbabka@suse.com>]

On 2/20/26 18:35, Marcelo Tosatti wrote:
> 
> Only call rcu_free_sheaf_nobarn if pcs->rcu_free is not NULL.
> 
> So it seems safe?

I guess it is.

>> How would this work with houskeeping on return to userspace approach?
>> 
>> - Would we just walk the list of all caches to flush them? could be
>> expensive. Would we somehow note only those that need it? That would make
>> the fast paths do something extra?
>> 
>> - If some other CPU executed kmem_cache_destroy(), it would have to wait for
>> the isolated cpu returning to userspace. Do we have the means for
>> synchronizing on that? Would that risk a deadlock? We used to have a
>> deferred finishing of the destroy for other reasons but were glad to get rid
>> of it when it was possible, now it might be necessary to revive it?
> 
> I don't think you can expect system calls to return to userspace in 
> a given amount of time. Could be in kernel mode for long periods of
> time.
> 
>> How would this work with QPW?
>> 
>> - probably fast paths more expensive due to spin lock vs local_trylock_t
>> 
>> - flush_rcu_sheaves_on_cache() needs to be solved safely (see above)
>> 
>> What if we avoid percpu sheaves completely on isolated cpus and instead
>> allocate/free using the slowpaths?
>> 
>> - It could probably be achieved without affecting fastpaths, as we already
>> handle bootstrap without sheaves, so it's implemented in a way to not affect
>> fastpaths.
>> 
>> - Would it slow the isolcpu workloads down too much when they do a syscall?
>>   - compared to "houskeeping on return to userspace" flushing, maybe not?
>> Because in that case the syscall starts with sheaves flushed from previous
>> return, it has to do something expensive to get the initial sheaf, then
>> maybe will use only on or few objects, then on return has to flush
>> everything. Likely the slowpath might be faster, unless it allocates/frees
>> many objects from the same cache.
>>   - compared to QPW - it would be slower as QPW would mostly retain sheaves
>> populated, the need for flushes should be very rare
>> 
>> So if we can assume that workloads on isolated cpus make syscalls only
>> rarely, and when they do they can tolerate them being slower, I think the
>> "avoid sheaves on isolated cpus" would be the best way here.
> 
> I am not sure its safe to assume that. Ask Gemini about isolcpus use
> cases and:

I don't think it's answering the question about syscalls. But didn't read
too closely given the nature of it.

> 
> For example, AF_XDP bypass uses system calls (and wants isolcpus):
> 
> https://www.quantvps.com/blog/kernel-bypass-in-hft?srsltid=AfmBOoryeSxuuZjzTJIC9O-Ag8x4gSwjs-V4Xukm2wQpGmwDJ6t4szuE

Didn't spot system calls mentioned TBH.