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

[Peter Zijlstra <peterz@infradead.org>]

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?

It can lead to try to priority boost the idle thread, among other crazy
things.