Re: [PATCH] procfs: add syscall statistics

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, May 27, 2022 at 07:09:59PM +0800, Zhang Yuchen wrote:
> Add /proc/syscalls to display percpu syscall count.
> 
> We need a less resource-intensive way to count syscall per cpu
> for system problem location.

Why?

How is this less resource intensive than perf?

> There is a similar utility syscount in the BCC project, but syscount
> has a high performance cost.

What is that cost?

> The following is a comparison on the same machine, using UnixBench
> System Call Overhead:
> 
>     ┏━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━┳━━━━━━━━┓
>     ┃ Change        ┃ Unixbench Score ┃ Loss   ┃
>     ┡━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━╇━━━━━━━━┩
>     │ no change     │ 1072.6          │ ---    │
>     │ syscall count │ 982.5           │ 8.40%  │
>     │ bpf syscount  │ 614.2           │ 42.74% │
>     └───────────────┴─────────────────┴────────┘

Again, what about perf?

> UnixBench System Call Use sys_gettid to test, this system call only reads
> one variable, so the performance penalty seems large. When tested with
> fork, the test scores were almost the same.
> 
> So the conclusion is that it does not have a significant impact on system
> call performance.

8% is huge for a system-wide decrease in performance.  Who would ever
use this?

> This function depends on CONFIG_FTRACE_SYSCALLS because the system call
> number is stored in syscall_metadata.
> 
> Signed-off-by: Zhang Yuchen <zhangyuchen.lcr@bytedance.com>
> ---
>  Documentation/filesystems/proc.rst       | 28 +++++++++
>  arch/arm64/include/asm/syscall_wrapper.h |  2 +-
>  arch/s390/include/asm/syscall_wrapper.h  |  4 +-
>  arch/x86/include/asm/syscall_wrapper.h   |  2 +-
>  fs/proc/Kconfig                          |  7 +++
>  fs/proc/Makefile                         |  1 +
>  fs/proc/syscall.c                        | 79 ++++++++++++++++++++++++
>  include/linux/syscalls.h                 | 51 +++++++++++++--
>  8 files changed, 165 insertions(+), 9 deletions(-)
>  create mode 100644 fs/proc/syscall.c
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 1bc91fb8c321..80394a98a192 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -686,6 +686,7 @@ files are there, and which are missing.
>   fs 	      File system parameters, currently nfs/exports	(2.4)
>   ide          Directory containing info about the IDE subsystem
>   interrupts   Interrupt usage
> + syscalls     Syscall count for each cpu
>   iomem 	      Memory map					(2.4)
>   ioports      I/O port usage
>   irq 	      Masks for irq to cpu affinity			(2.4)(smp?)
> @@ -1225,6 +1226,33 @@ Provides counts of softirq handlers serviced since boot time, for each CPU.
>      HRTIMER:         0          0          0          0
>  	RCU:      1678       1769       2178       2250
>  
> +syscalls
> +~~~~~~~~
> +
> +Provides counts of syscall since boot time, for each cpu.
> +
> +::
> +
> +    > cat /proc/syscalls
> +               CPU0       CPU1       CPU2       CPU3
> +      0:       3743       3099       3770       3242   sys_read
> +      1:        222        559        822        522   sys_write
> +      2:          0          0          0          0   sys_open
> +      3:       6481      18754      12077       7349   sys_close
> +      4:      11362      11120      11343      10665   sys_newstat
> +      5:       5224      13880       8578       5971   sys_newfstat
> +      6:       1228       1269       1459       1508   sys_newlstat
> +      7:         90         43         64         67   sys_poll
> +      8:       1635       1000       2071       1161   sys_lseek
> +    .... omit the middle line ....
> +    441:          0          0          0          0   sys_epoll_pwait2
> +    442:          0          0          0          0   sys_mount_setattr
> +    443:          0          0          0          0   sys_quotactl_fd
> +    447:          0          0          0          0   sys_memfd_secret
> +    448:          0          0          0          0   sys_process_mrelease
> +    449:          0          0          0          0   sys_futex_waitv
> +    450:          0          0          0          0   sys_set_mempolicy_home_node

So for systems with large numbers of CPUs, these are huge lines?  Have
you tested this on large systems?  If so, how big?

thanks,

greg k-h