On Tue, Dec 13, 2022 at 10:48:43AM -0800, Song Liu wrote: > On Tue, Dec 13, 2022 at 6:09 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > Both bpf_trace_printk and bpf_trace_vprintk helpers use static buffer > > guarded with trace_printk_lock spin lock. > > > > The spin lock contention causes issues with bpf programs attached to > > contention_begin tracepoint [1] [2]. > > > > Andrii suggested we could get rid of the contention by using trylock, > > but we could actually get rid of the spinlock completely by using > > percpu buffers the same way as for bin_args in bpf_bprintf_prepare > > function. > > > > Adding 4 per cpu buffers (1k each) which should be enough for all > > possible nesting contexts (normal, softirq, irq, nmi) or possible > > (yet unlikely) probe within the printk helpers. > > > > In very unlikely case we'd run out of the nesting levels the printk > > will be omitted. > > > > [1] https://lore.kernel.org/bpf/CACkBjsakT_yWxnSWr4r-0TpPvbKm9-OBmVUhJb7hV3hY8fdCkw@xxxxxxxxxxxxxx/ > > [2] https://lore.kernel.org/bpf/CACkBjsaCsTovQHFfkqJKto6S4Z8d02ud1D7MPESrHa1cVNNTrw@xxxxxxxxxxxxxx/ > > > > Reported-by: Hao Sun <sunhao.th@xxxxxxxxx> > > Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> > > --- > > kernel/trace/bpf_trace.c | 61 +++++++++++++++++++++++++++++++--------- > > 1 file changed, 47 insertions(+), 14 deletions(-) > > > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > > index 3bbd3f0c810c..b9287b3a5540 100644 > > --- a/kernel/trace/bpf_trace.c > > +++ b/kernel/trace/bpf_trace.c > > @@ -369,33 +369,62 @@ static const struct bpf_func_proto *bpf_get_probe_write_proto(void) > > return &bpf_probe_write_user_proto; > > } > > > > -static DEFINE_RAW_SPINLOCK(trace_printk_lock); > > - > > #define MAX_TRACE_PRINTK_VARARGS 3 > > #define BPF_TRACE_PRINTK_SIZE 1024 > > +#define BPF_TRACE_PRINTK_LEVELS 4 > > + > > +struct trace_printk_buf { > > + char data[BPF_TRACE_PRINTK_LEVELS][BPF_TRACE_PRINTK_SIZE]; > > + int level; > > +}; > > +static DEFINE_PER_CPU(struct trace_printk_buf, printk_buf); > > + > > +static void put_printk_buf(struct trace_printk_buf __percpu *buf) > > +{ > > + if (WARN_ON_ONCE(this_cpu_read(buf->level) == 0)) > > + return; > > + this_cpu_dec(buf->level); > > + preempt_enable(); > > +} > > + > > +static bool get_printk_buf(struct trace_printk_buf __percpu *buf, char **data) > > +{ > > + int level; > > + > > + preempt_disable(); > > Can we use migrate_disable() instead? I think that should work.. while checking on that I found comment in in include/linux/preempt.h (though dated): The end goal must be to get rid of migrate_disable but looks like both should work here and there are trade offs for using each of them > > > + level = this_cpu_inc_return(buf->level); > > + if (level > BPF_TRACE_PRINTK_LEVELS) { > > Maybe add WARN_ON_ONCE() here? ok, will add thanks, jirka
