Re: [PATCH v2 net-next 2/2] mm/vmpressure: add tracepoint for socket pressure detection

[Daniel Sedlak <daniel.sedlak@cdn77.com>]

Hi Kuniyuki,

On 7/14/25 8:02 PM, Kuniyuki Iwashima wrote:
>> +TRACE_EVENT(memcg_socket_under_pressure,
>> +
>> +       TP_PROTO(const struct mem_cgroup *memcg, unsigned long scanned,
>> +               unsigned long reclaimed),
>> +
>> +       TP_ARGS(memcg, scanned, reclaimed),
>> +
>> +       TP_STRUCT__entry(
>> +               __field(u64, id)
>> +               __field(unsigned long, scanned)
>> +               __field(unsigned long, reclaimed)
>> +       ),
>> +
>> +       TP_fast_assign(
>> +               __entry->id = cgroup_id(memcg->css.cgroup);
>> +               __entry->scanned = scanned;
>> +               __entry->reclaimed = reclaimed;
>> +       ),
>> +
>> +       TP_printk("memcg_id=%llu scanned=%lu reclaimed=%lu",
>> +               __entry->id,
> 
> Maybe a noob question: How can we translate the memcg ID
> to the /sys/fs/cgroup/... path ?

IMO this should be really named `cgroup_id` instead of `memcg_id`, but 
we kept the latter to keep consistency with the rest of the file.

To find cgroup path you can use:
- find /sys/fs/cgroup/ -inum `memcg_id`, and it will print "path" to the 
affected cgroup.
- or you can use bpftrace tracepoint hooks and there is a helper 
function [1].

Or we can put the cgroup_path to the tracepoint instead of that ID, but 
I feel it can be too much overhead, the paths can be pretty long.

Link: https://bpftrace.org/docs/latest#functions-cgroup_path [1]
> It would be nice to place this patch first and the description of
> patch 2 has how to use the new stat with this tracepoint.

Sure, can do that. However, I am unsure how a good idea is to 
cross-reference commits, since each may go through a different tree 
because each commit is for a different subsystem. They would have to go 
through one tree, right?


>> +               __entry->scanned,
>> +               __entry->reclaimed)
>> +);
>> +
>>   #endif /* _TRACE_MEMCG_H */
>>
>>   /* This part must be outside protection */
>> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
>> index bd5183dfd879..aa9583066731 100644
>> --- a/mm/vmpressure.c
>> +++ b/mm/vmpressure.c
>> @@ -21,6 +21,8 @@
>>   #include <linux/printk.h>
>>   #include <linux/vmpressure.h>
>>
>> +#include <trace/events/memcg.h>
>> +
>>   /*
>>    * The window size (vmpressure_win) is the number of scanned pages before
>>    * we try to analyze scanned/reclaimed ratio. So the window is used as a
>> @@ -317,6 +319,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>>                           * pressure events can occur.
>>                           */
>>                          WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
>> +                       trace_memcg_socket_under_pressure(memcg, scanned, reclaimed);
> 
> This is triggered only when we enter the memory pressure state
> and not when we leave the state, right ?  Is it possible to issue
> such an event ?

AFAIK, the currently used API in the vmpressure function does not have 
anything like enter or leave the socket memory pressure state. It only 
periodically re-arms the socket pressure for a specific duration. So the 
current tracepoint is called when the socket pressure is re-armed.

I am not sure how feasible it is to rewrite it so we have enter and 
leave logic. I am not that familiar with those code paths.

Thanks!
Daniel