On Tue, Dec 13, 2022 at 03:52:38PM -0800, Yonghong Song wrote: > > > On 12/13/22 1:53 PM, Jiri Olsa wrote: > > 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> > > Maybe change to subject to 'Remove trace_printk_lock' instead > of 'Remove trace_printk_lock lock'? The 'trace_printk_lock' > should already imply 'lock'? ok > > > > > --- > > > > 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): > > I am not sure about whether migrate_disable() will work. For example, > . task1 takes over level=0 buffer, level = 1 > . task1 yields to task2 with preemption in the same cpu > . task2 takes over level=1 buffer, level = 2 > . task2 yields to task1 in the same cpu > . task1 releases the buffer, level = 1 > . task1 yields to task3 in the same cpu > . task3 takes over level=1 buffer, level = 2 > <=== we have an issue here, both task2 and task3 use level=1 buffer. hum, did not think of that.. will keep the preempt_disable then thanks, jirka > > > > > 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
