Re: [RFC PATCH 08/13] mm/kwatch: add hardware breakpoint backend

[Jinchao Wang <wangjinchao600@gmail.com>]

On 7/14/2026 5:14 PM, Steven Rostedt wrote:
> On Wed, 15 Jul 2026 02:32:06 +0800
> Jinchao Wang <wangjinchao600@gmail.com> wrote:
> 
>> --- /dev/null
>> +++ b/include/trace/events/kwatch.h
>> @@ -0,0 +1,57 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#undef TRACE_SYSTEM
>> +#define TRACE_SYSTEM kwatch
>> +
>> +#if !defined(_TRACE_KWATCH_H) || defined(TRACE_HEADER_MULTI_READ)
>> +#define _TRACE_KWATCH_H
>> +
>> +#include <linux/tracepoint.h>
>> +#include <linux/ptrace.h>
>> +
>> +#define KWATCH_STACK_DEPTH 8
>> +
>> +struct trace_seq;
>> +const char *kwatch_trace_print_stack(struct trace_seq *p,
>> +				     const unsigned long *stack,
>> +				     unsigned int nr);
>> +
>> +TRACE_EVENT(kwatch_hit,
>> +	TP_PROTO(unsigned long ip, unsigned long sp, unsigned long addr,
>> +		 u64 time_ns,
>> +		 unsigned long *stack_entries, unsigned int stack_nr),
>> +	TP_ARGS(ip, sp, addr, time_ns, stack_entries, stack_nr),
>> +
>> +	TP_STRUCT__entry(
>> +		__field(unsigned long, ip)
>> +		__field(unsigned long, sp)
>> +		__field(unsigned long, addr)
>> +		__field(u64, time_ns)
> 
> Move the time_ns to the first field, as unsigned long on 32 bit
> architectures is 4 bytes, and this will make 4 byte "hole" in the event.
Will fix in v2.> 
> 
>> +		__field(unsigned int, stack_nr)
> 
> Make stack_nr the last element for the same reason.

Will fix in v2.

> 
>> +		__array(unsigned long, stack, KWATCH_STACK_DEPTH)
> 
> Make the above a dynamic array based on stack entries.
> 
> 		__dynamic_array(unsigned long, stack, min_t(unsigned int, stack_nr,
> 			  KWATCH_STACK_DEPTH);

Much better than always paying for the full depth - will convert to
__dynamic_array (and use __get_dynamic_array() in TP_fast_assign and
TP_printk as you showed) in v2.

Thank you for the review!

Thanks,
Jinchao

> 
> 
>> +	),
>> +
>> +	TP_fast_assign(
>> +		unsigned int i;
> 		unsigned long *stack = __get_dynamic_array(stack);
>> +
>> +		__entry->ip = ip;
>> +		__entry->sp = sp;
>> +		__entry->addr = addr;
>> +		__entry->time_ns = time_ns;
>> +		__entry->stack_nr = min_t(unsigned int, stack_nr,
>> +					  KWATCH_STACK_DEPTH);
>> +		for (i = 0; i < __entry->stack_nr; i++)
>> +			__entry->stack[i] = stack_entries[i];
> 
> 			stack[i] = stack_entries[i];
> 
>> +	),
>> +
>> +	TP_printk("KWatch HIT: time=%llu.%06lu ip=%pS addr=0x%lx%s",
>> +		  __entry->time_ns / 1000000000ULL,
>> +		  (unsigned long)((__entry->time_ns / 1000ULL) % 1000000ULL),
>> +		  (void *)__entry->ip, __entry->addr,
>> +		  kwatch_trace_print_stack(p, __entry->stack,
> 
> 		  kwatch_trace_print_stack(p, __get_dynamic_array(stack),
> 
>> +					   __entry->stack_nr))
>> +);
>> +
> 
> -- Steve