Re: [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code

[Alexei Starovoitov]

On Wed, Feb 19, 2020 at 01:49:57AM +0100, Thomas Gleixner wrote:
> Alexei,
> 
> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
> > Overall looks great.
> > Thank you for taking time to write commit logs and detailed cover letter.
> > I think s/__this_cpu_inc/this_cpu_inc/ is the only bit that needs to be
> > addressed for it to be merged.
> > There were few other suggestions from Mathieu and Jakub.
> > Could you address them and resend?
> 
> I have them fixed up already, but I was waiting for further
> comments. I'll send it out tomorrow morning as I'm dead tired by now.
> 
> > I saw patch 1 landing in tip tree, but it needs to be in bpf-next as well
> > along with the rest of the series. Does it really need to be in the tip?
> > I would prefer to take the whole thing and avoid conflicts around
> > migrate_disable() especially if nothing in tip is going to use it in this
> > development cycle. So just drop patch 1 from the tip?
> 
> I'll add patch 2 to a tip branch as well and I'll give you a tag to pull
> into BPF (which has only those two commits). 

That works too.

> > Regarding
> > union {
> >    raw_spinlock_t  raw_lock;
> >    spinlock_t      lock;
> > };
> > yeah. it's not pretty, but I also don't have better ideas.
> 
> Yeah. I really tried hard to avoid it, but the alternative solution was
> code duplication which was even more horrible.
> 
> > Regarding migrate_disable()... can you enable it without the rest of RT?
> > I haven't seen its implementation. I suspect it's scheduler only change?
> > If I can use migrate_disable() without RT it will help my work on sleepable
> > BPF programs. I would only have to worry about rcu_read_lock() since
> > preempt_disable() is nicely addressed.
> 
> You have to talk to Peter Zijlstra about this as this is really
> scheduler relevant stuff. FYI, he undamentaly hates migrate_disable()
> from a schedulabilty POV, but as with the above lock construct the
> amount of better solutions is also close to zero.

I would imagine that migrate_disable is like temporary cpu pinning. The
scheduler has to have all the logic to make scheduling decisions quickly in
presence of pinned tasks and additional migrate_disable shouldn't introduce
slowdowns or complexity to critical path. Anyway, we'll discuss further
when migrate_disable patches hit mailing lists.