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? > + level = this_cpu_inc_return(buf->level); > + if (level > BPF_TRACE_PRINTK_LEVELS) { Maybe add WARN_ON_ONCE() here? Thanks, Song [...]
