Message-ID: <20231213214632.15047c40@gandalf.local.home>
Date:   Wed, 13 Dec 2023 21:46:32 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] ring-buffer: Remove 32bit timestamp logic

Linus,

Looking for some advice on this.

tl;dr;  The ring-buffer timestamp requires a 64-bit cmpxchg to keep the
timestamps in sync (only in the slow paths). I was told that 64-bit cmpxchg
can be extremely slow on 32-bit architectures. So I created a rb_time_t
that on 64-bit was a normal local64_t type, and on 32-bit it's represented
by 3 32-bit words and a counter for synchronization. But this now requires
three 32-bit cmpxchgs for where one simple 64-bit cmpxchg would do.

Not having any 32-bit hardware to test on, I simply push through this
complex code for the 32-bit case. I tested it on both 32-bit (running on a
x86_64 machine) and 64-bit kernels and it seemed rather robust.

But now that I'm doing some heavy development on the ring buffer again, I
came across a few very subtle bugs in this code (and so has Mathieu Desnoyers).
We started discussing how much time this is actually saving to be worth the
complexity, and actually found some hardware to test. One Atom processor.

For the Atom it was 11.5ns for 32-bit and 16ns for 64-bit.

Granted, this isn't being contended on. But a 30% improvement doesn't
justify 3 to 1 cmpxchgs.

I plan on just nuking the whole thing (the patch below), which is basically
a revert of 10464b4aa605e ("ring-buffer: Add rb_time_t 64 bit operations for
speeding up 32 bit").

Now my question to you. Should I bother with pushing to you the subtle
fixes to this code and send you the revert for the next merge window, or do
you think I should just say "screw it" and nuke it now?

Or do you think it's worth keeping for some other architecture that 3
32-bit cmpxchgs are still faster than a single 64-bit one?

Thanks,

-- Steve



On Wed, 13 Dec 2023 21:11:26 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> Each event has a 27 bit timestamp delta that is used to hold the delta
> from the last event. If the time between events is greater than 2^27, then
> a timestamp is added that holds a 59 bit absolute timestamp.
> 
> Until a389d86f7fd09 ("ring-buffer: Have nested events still record running
> time stamp"), if an interrupt interrupted an event in progress, all the
> events delta would be zero to not deal with the races that need to be
> handled. The commit a389d86f7fd09 changed that to handle the races giving
> all events, even those that preempt other events, still have an accurate
> timestamp.
> 
> To handle those races requires performing 64-bit cmpxchg on the
> timestamps. But doing 64-bit cmpxchg on 32-bit architectures is considered
> very slow. To try to deal with this the timestamp logic was broken into
> two and then three 32-bit cmpxchgs, with the thought that two (or three)
> 32-bit cmpxchgs are still faster than a single 64-bit cmpxchg on 32-bit
> architectures.
> 
> Part of the problem with this is that I didn't have any 32-bit
> architectures to test on. After hitting several subtle bugs in this code,
> an effort was made to try and see if three 32-bit cmpxchgs are indeed
> faster than a single 64-bit. After a few people brushed off the dust of
> their old 32-bit machines, tests were done, and even though 32-bit cmpxchg
> was faster than a single 64-bit, it was in the order of 50% at best, not
> 300%.
> 
> This means that this complex code is not only complex but also not even
> faster than just using 64-bit cmpxchg.
> 
> Nuke it!
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> [
>    Should we just push this now and mark it for stable?
>    That is, just get rid of this logic for all kernels.
> ]
>  kernel/trace/ring_buffer.c | 226 ++-----------------------------------
>  1 file changed, 12 insertions(+), 214 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 3fce5e4b4f2b..b05416e14cb6 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -27,6 +27,7 @@
>  #include <linux/cpu.h>
>  #include <linux/oom.h>
>  
> +#include <asm/local64.h>
>  #include <asm/local.h>
>  
>  /*
> @@ -463,27 +464,9 @@ enum {
>  	RB_CTX_MAX
>  };
>  
> -#if BITS_PER_LONG == 32
> -#define RB_TIME_32
> -#endif
> -
> -/* To test on 64 bit machines */
> -//#define RB_TIME_32
> -
> -#ifdef RB_TIME_32
> -
> -struct rb_time_struct {
> -	local_t		cnt;
> -	local_t		top;
> -	local_t		bottom;
> -	local_t		msb;
> -};
> -#else
> -#include <asm/local64.h>
>  struct rb_time_struct {
>  	local64_t	time;
>  };
> -#endif
>  typedef struct rb_time_struct rb_time_t;
>  
>  #define MAX_NEST	5
> @@ -573,183 +556,9 @@ struct ring_buffer_iter {
>  	int				missed_events;
>  };
>  
> -#ifdef RB_TIME_32
> -
> -/*
> - * On 32 bit machines, local64_t is very expensive. As the ring
> - * buffer doesn't need all the features of a true 64 bit atomic,
> - * on 32 bit, it uses these functions (64 still uses local64_t).
> - *
> - * For the ring buffer, 64 bit required operations for the time is
> - * the following:
> - *
> - *  - Reads may fail if it interrupted a modification of the time stamp.
> - *      It will succeed if it did not interrupt another write even if
> - *      the read itself is interrupted by a write.
> - *      It returns whether it was successful or not.
> - *
> - *  - Writes always succeed and will overwrite other writes and writes
> - *      that were done by events interrupting the current write.
> - *
> - *  - A write followed by a read of the same time stamp will always succeed,
> - *      but may not contain the same value.
> - *
> - *  - A cmpxchg will fail if it interrupted another write or cmpxchg.
> - *      Other than that, it acts like a normal cmpxchg.
> - *
> - * The 60 bit time stamp is broken up by 30 bits in a top and bottom half
> - *  (bottom being the least significant 30 bits of the 60 bit time stamp).
> - *
> - * The two most significant bits of each half holds a 2 bit counter (0-3).
> - * Each update will increment this counter by one.
> - * When reading the top and bottom, if the two counter bits match then the
> - *  top and bottom together make a valid 60 bit number.
> - */
> -#define RB_TIME_SHIFT	30
> -#define RB_TIME_VAL_MASK ((1 << RB_TIME_SHIFT) - 1)
> -#define RB_TIME_MSB_SHIFT	 60
> -
> -static inline int rb_time_cnt(unsigned long val)
> -{
> -	return (val >> RB_TIME_SHIFT) & 3;
> -}
> -
> -static inline u64 rb_time_val(unsigned long top, unsigned long bottom)
> -{
> -	u64 val;
> -
> -	val = top & RB_TIME_VAL_MASK;
> -	val <<= RB_TIME_SHIFT;
> -	val |= bottom & RB_TIME_VAL_MASK;
> -
> -	return val;
> -}
> -
> -static inline bool __rb_time_read(rb_time_t *t, u64 *ret, unsigned long *cnt)
> -{
> -	unsigned long top, bottom, msb;
> -	unsigned long c;
> -
> -	/*
> -	 * If the read is interrupted by a write, then the cnt will
> -	 * be different. Loop until both top and bottom have been read
> -	 * without interruption.
> -	 */
> -	do {
> -		c = local_read(&t->cnt);
> -		top = local_read(&t->top);
> -		bottom = local_read(&t->bottom);
> -		msb = local_read(&t->msb);
> -	} while (c != local_read(&t->cnt));
> -
> -	*cnt = rb_time_cnt(top);
> -
> -	/* If top, msb or bottom counts don't match, this interrupted a write */
> -	if (*cnt != rb_time_cnt(msb) || *cnt != rb_time_cnt(bottom))
> -		return false;
> -
> -	/* The shift to msb will lose its cnt bits */
> -	*ret = rb_time_val(top, bottom) | ((u64)msb << RB_TIME_MSB_SHIFT);
> -	return true;
> -}
> -
> -static bool rb_time_read(rb_time_t *t, u64 *ret)
> -{
> -	unsigned long cnt;
> -
> -	return __rb_time_read(t, ret, &cnt);
> -}
> -
> -static inline unsigned long rb_time_val_cnt(unsigned long val, unsigned long cnt)
> -{
> -	return (val & RB_TIME_VAL_MASK) | ((cnt & 3) << RB_TIME_SHIFT);
> -}
> -
> -static inline void rb_time_split(u64 val, unsigned long *top, unsigned long *bottom,
> -				 unsigned long *msb)
> -{
> -	*top = (unsigned long)((val >> RB_TIME_SHIFT) & RB_TIME_VAL_MASK);
> -	*bottom = (unsigned long)(val & RB_TIME_VAL_MASK);
> -	*msb = (unsigned long)(val >> RB_TIME_MSB_SHIFT);
> -}
> -
> -static inline void rb_time_val_set(local_t *t, unsigned long val, unsigned long cnt)
> -{
> -	val = rb_time_val_cnt(val, cnt);
> -	local_set(t, val);
> -}
> -
> -static void rb_time_set(rb_time_t *t, u64 val)
> -{
> -	unsigned long cnt, top, bottom, msb;
> -
> -	rb_time_split(val, &top, &bottom, &msb);
> -
> -	/* Writes always succeed with a valid number even if it gets interrupted. */
> -	do {
> -		cnt = local_inc_return(&t->cnt);
> -		rb_time_val_set(&t->top, top, cnt);
> -		rb_time_val_set(&t->bottom, bottom, cnt);
> -		rb_time_val_set(&t->msb, val >> RB_TIME_MSB_SHIFT, cnt);
> -	} while (cnt != local_read(&t->cnt));
> -}
> -
> -static inline bool
> -rb_time_read_cmpxchg(local_t *l, unsigned long expect, unsigned long set)
> -{
> -	return local_try_cmpxchg(l, &expect, set);
> -}
> -
> -static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
> -{
> -	unsigned long cnt, top, bottom, msb;
> -	unsigned long cnt2, top2, bottom2, msb2;
> -	u64 val;
> -
> -	/* Any interruptions in this function should cause a failure */
> -	cnt = local_read(&t->cnt);
> -
> -	/* The cmpxchg always fails if it interrupted an update */
> -	 if (!__rb_time_read(t, &val, &cnt2))
> -		 return false;
> -
> -	 if (val != expect)
> -		 return false;
> -
> -	 if ((cnt & 3) != cnt2)
> -		 return false;
> -
> -	 cnt2 = cnt + 1;
> -
> -	 rb_time_split(val, &top, &bottom, &msb);
> -	 msb = rb_time_val_cnt(msb, cnt);
> -	 top = rb_time_val_cnt(top, cnt);
> -	 bottom = rb_time_val_cnt(bottom, cnt);
> -
> -	 rb_time_split(set, &top2, &bottom2, &msb2);
> -	 msb2 = rb_time_val_cnt(msb2, cnt);
> -	 top2 = rb_time_val_cnt(top2, cnt2);
> -	 bottom2 = rb_time_val_cnt(bottom2, cnt2);
> -
> -	if (!rb_time_read_cmpxchg(&t->cnt, cnt, cnt2))
> -		return false;
> -	if (!rb_time_read_cmpxchg(&t->msb, msb, msb2))
> -		return false;
> -	if (!rb_time_read_cmpxchg(&t->top, top, top2))
> -		return false;
> -	if (!rb_time_read_cmpxchg(&t->bottom, bottom, bottom2))
> -		return false;
> -	return true;
> -}
> -
> -#else /* 64 bits */
> -
> -/* local64_t always succeeds */
> -
> -static inline bool rb_time_read(rb_time_t *t, u64 *ret)
> +static inline void rb_time_read(rb_time_t *t, u64 *ret)
>  {
>  	*ret = local64_read(&t->time);
> -	return true;
>  }
>  static void rb_time_set(rb_time_t *t, u64 val)
>  {
> @@ -760,7 +569,6 @@ static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
>  {
>  	return local64_try_cmpxchg(&t->time, &expect, set);
>  }
> -#endif
>  
>  /*
>   * Enable this to make sure that the event passed to
> @@ -867,10 +675,7 @@ u64 ring_buffer_event_time_stamp(struct trace_buffer *buffer,
>  	WARN_ONCE(1, "nest (%d) greater than max", nest);
>  
>   fail:
> -	/* Can only fail on 32 bit */
> -	if (!rb_time_read(&cpu_buffer->write_stamp, &ts))
> -		/* Screw it, just read the current time */
> -		ts = rb_time_stamp(cpu_buffer->buffer);
> +	rb_time_read(&cpu_buffer->write_stamp, &ts);
>  
>  	return ts;
>  }
> @@ -2867,7 +2672,7 @@ rb_check_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
>  		  (unsigned long long)info->ts,
>  		  (unsigned long long)info->before,
>  		  (unsigned long long)info->after,
> -		  (unsigned long long)(rb_time_read(&cpu_buffer->write_stamp, &write_stamp) ? write_stamp : 0),
> +		  (unsigned long long)({rb_time_read(&cpu_buffer->write_stamp, &write_stamp); write_stamp;}),
>  		  sched_clock_stable() ? "" :
>  		  "If you just came from a suspend/resume,\n"
>  		  "please switch to the trace global clock:\n"
> @@ -3025,8 +2830,7 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
>  
>  	delta = rb_time_delta(event);
>  
> -	if (!rb_time_read(&cpu_buffer->write_stamp, &write_stamp))
> -		return false;
> +	rb_time_read(&cpu_buffer->write_stamp, &write_stamp);
>  
>  	/* Make sure the write stamp is read before testing the location */
>  	barrier();
> @@ -3569,16 +3373,14 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  	struct ring_buffer_event *event;
>  	struct buffer_page *tail_page;
>  	unsigned long tail, write, w;
> -	bool a_ok;
> -	bool b_ok;
>  
>  	/* Don't let the compiler play games with cpu_buffer->tail_page */
>  	tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
>  
>   /*A*/	w = local_read(&tail_page->write) & RB_WRITE_MASK;
>  	barrier();
> -	b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
> -	a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
> +	rb_time_read(&cpu_buffer->before_stamp, &info->before);
> +	rb_time_read(&cpu_buffer->write_stamp, &info->after);
>  	barrier();
>  	info->ts = rb_time_stamp(cpu_buffer->buffer);
>  
> @@ -3593,7 +3395,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  		if (!w) {
>  			/* Use the sub-buffer timestamp */
>  			info->delta = 0;
> -		} else if (unlikely(!a_ok || !b_ok || info->before != info->after)) {
> +		} else if (unlikely(info->before != info->after)) {
>  			info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
>  			info->length += RB_LEN_TIME_EXTEND;
>  		} else {
> @@ -3622,13 +3424,11 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  
>  	if (likely(tail == w)) {
>  		u64 save_before;
> -		bool s_ok;
>  
>  		/* Nothing interrupted us between A and C */
>   /*D*/		rb_time_set(&cpu_buffer->write_stamp, info->ts);
>  		barrier();
> - /*E*/		s_ok = rb_time_read(&cpu_buffer->before_stamp, &save_before);
> -		RB_WARN_ON(cpu_buffer, !s_ok);
> + /*E*/		rb_time_read(&cpu_buffer->before_stamp, &save_before);
>  		if (likely(!(info->add_timestamp &
>  			     (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
>  			/* This did not interrupt any time update */
> @@ -3641,8 +3441,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  		if (unlikely(info->ts != save_before)) {
>  			/* SLOW PATH - Interrupted between C and E */
>  
> -			a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
> -			RB_WARN_ON(cpu_buffer, !a_ok);
> +			rb_time_read(&cpu_buffer->write_stamp, &info->after);
>  
>  			/* Write stamp must only go forward */
>  			if (save_before > info->after) {
> @@ -3657,9 +3456,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  	} else {
>  		u64 ts;
>  		/* SLOW PATH - Interrupted between A and C */
> -		a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
> -		/* Was interrupted before here, write_stamp must be valid */
> -		RB_WARN_ON(cpu_buffer, !a_ok);
> +		rb_time_read(&cpu_buffer->write_stamp, &info->after);
>  		ts = rb_time_stamp(cpu_buffer->buffer);
>  		barrier();
>   /*E*/		if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
> @@ -3720,6 +3517,7 @@ rb_reserve_next_event(struct trace_buffer *buffer,
>  	struct rb_event_info info;
>  	int nr_loops = 0;
>  	int add_ts_default;
> +	static int once;
>  
>  	/* ring buffer does cmpxchg, make sure it is safe in NMI context */
>  	if (!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) &&

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