Re: [PATCH bpf-next v4 1/6] mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation

[Michal Hocko <mhocko@suse.com>]

On Tue 14-01-25 11:39:46, Peter Zijlstra wrote:
> On Tue, Jan 14, 2025 at 11:19:41AM +0100, Michal Hocko wrote:
> > On Tue 14-01-25 10:53:55, Peter Zijlstra wrote:
> > > On Mon, Jan 13, 2025 at 06:19:17PM -0800, Alexei Starovoitov wrote:
> > > > From: Alexei Starovoitov <ast@kernel.org>
> > > > 
> > > > Tracing BPF programs execute from tracepoints and kprobes where
> > > > running context is unknown, but they need to request additional
> > > > memory.
> > > 
> > > > The prior workarounds were using pre-allocated memory and
> > > > BPF specific freelists to satisfy such allocation requests.
> > > > Instead, introduce gfpflags_allow_spinning() condition that signals
> > > > to the allocator that running context is unknown.
> > > > Then rely on percpu free list of pages to allocate a page.
> > > > The rmqueue_pcplist() should be able to pop the page from.
> > > > If it fails (due to IRQ re-entrancy or list being empty) then
> > > > try_alloc_pages() attempts to spin_trylock zone->lock
> > > > and refill percpu freelist as normal.
> > > 
> > > > BPF program may execute with IRQs disabled and zone->lock is
> > > > sleeping in RT, so trylock is the only option. 
> > > 
> > > how is spin_trylock() from IRQ context not utterly broken in RT?
> > 
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> > +		return NULL;
> > 
> > Deals with that, right?
> 
> Changelog didn't really mention that, did it? -- it seems to imply quite
> the opposite :/

yes, one has to look into the implementation see all the constrains and
the changelog could/should be improved in that regards.

> But maybe, I suppose any BPF program needs to expect failure due to this
> being trylock. I just worry some programs will malfunction due to never
> succeeding -- and RT getting blamed for this.

This is a good question. The current implementation has fewer constrains
and there are no data points about the success rate with the new
implementation. But to be completely honest I am not really sure how
much BPF is used on PREEMPT_RT systems and with RT workloads so I am not
sure how much of a practical problem that is.

> 
> Maybe I worry too much.

-- 
Michal Hocko
SUSE Labs