Re: [PATCH 1/3] tracing: Remove dependency of saved_cmdlines_buffer on PID_MAX_DEFAULT

[Michal Koutný]

On Tue, Apr 09, 2024 at 11:01:26AM GMT, Steven Rostedt  
wrote:
> > -   tpid = pid & (PID_MAX_DEFAULT - 1);
> > +   tpid = pid % PID_MAP_SIZE;
> 
> Does that compile to the same? This is a fast path.

I didn't check.
If fast is the intetion, I would change it to something
like BUILD_BUG_ON(!(PID_MAP_SIZE % 2)) and keep the bit operation
without reliance on compiler optimizations.

Thanks for the response (I may not follow up on this single commit
though).

Michal


signature.asc
Description: PGP signature

Re: [PATCH 1/3] tracing: Remove dependency of saved_cmdlines_buffer on PID_MAX_DEFAULT

[Steven Rostedt]

On Mon,  8 Apr 2024 16:58:17 +0200
Michal Koutný  wrote:

> @@ -294,7 +295,7 @@ static void __trace_find_cmdline(int pid, char comm[])
>   return;
>   }
>  
> - tpid = pid & (PID_MAX_DEFAULT - 1);
> + tpid = pid % PID_MAP_SIZE;

Does that compile to the same? This is a fast path.

-- Steve


>   map = savedcmd->map_pid_to_cmdline[tpid];
>   if (map != NO_CMDLINE_MAP) {
>   tpid = savedcmd->map_cmdline_to_pid[map];

[PATCH 1/3] tracing: Remove dependency of saved_cmdlines_buffer on PID_MAX_DEFAULT

[Michal Koutný]

Calculations into map_pid_to_cmdline use PID_MAX_DEFAULT but they
actually depend on the size of map_pid_to_cmdline. The size of the map
may be arbitrary. First, refer to the map size where necessary, second,
pick a good value for the size of the map.
Since the buffer is allocated at boot (i.e. user cannot affect its size
later), accounting for full PID_MAX_LIMIT would inflate map's size
unnecessarily (4*4M) for all users. Stick to the original value of
4*32k, the commit 785e3c0a3a87 ("tracing: Map all PIDs to command
lines") explains why it still works for higher pids.

The point of this exercise is to remove dependency on PID_MAX_DEFAULT.

Signed-off-by: Michal Koutný 
---
 kernel/trace/trace_sched_switch.c | 11 ++-
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_sched_switch.c 
b/kernel/trace/trace_sched_switch.c
index 8a407adb0e1c..aca2dafdd97a 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -161,6 +161,7 @@ static size_t tgid_map_max;
 
 #define SAVED_CMDLINES_DEFAULT 128
 #define NO_CMDLINE_MAP UINT_MAX
+#define PID_MAP_SIZE (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)
 /*
  * Preemption must be disabled before acquiring trace_cmdline_lock.
  * The various trace_arrays' max_lock must be acquired in a context
@@ -168,7 +169,7 @@ static size_t tgid_map_max;
  */
 static arch_spinlock_t trace_cmdline_lock = __ARCH_SPIN_LOCK_UNLOCKED;
 struct saved_cmdlines_buffer {
-   unsigned map_pid_to_cmdline[PID_MAX_DEFAULT+1];
+   unsigned map_pid_to_cmdline[PID_MAP_SIZE];
unsigned *map_cmdline_to_pid;
unsigned cmdline_num;
int cmdline_idx;
@@ -248,7 +249,7 @@ int trace_save_cmdline(struct task_struct *tsk)
if (!tsk->pid)
return 1;
 
-   tpid = tsk->pid & (PID_MAX_DEFAULT - 1);
+   tpid = tsk->pid % PID_MAP_SIZE;
 
/*
 * It's not the end of the world if we don't get
@@ -294,7 +295,7 @@ static void __trace_find_cmdline(int pid, char comm[])
return;
}
 
-   tpid = pid & (PID_MAX_DEFAULT - 1);
+   tpid = pid % PID_MAP_SIZE;
map = savedcmd->map_pid_to_cmdline[tpid];
if (map != NO_CMDLINE_MAP) {
tpid = savedcmd->map_cmdline_to_pid[map];
@@ -645,8 +646,8 @@ tracing_saved_cmdlines_size_write(struct file *filp, const 
char __user *ubuf,
if (ret)
return ret;
 
-   /* must have at least 1 entry or less than PID_MAX_DEFAULT */
-   if (!val || val > PID_MAX_DEFAULT)
+   /* must have at least 1 entry or fit into map_pid_to_cmdline */
+   if (!val || val >= PID_MAP_SIZE)
return -EINVAL;
 
ret = tracing_resize_saved_cmdlines((unsigned int)val);
-- 
2.44.0