From: Stuart Henderson <stu@spacehopper.org>
Subject: Re: btrace, identifying syscall+0x5c4
To: tech <tech@openbsd.org>
Date: Mon, 2 Dec 2024 14:35:22 +0000

On 2024/12/02 15:12, Claudio Jeker wrote:
> On Mon, Dec 02, 2024 at 01:53:02PM +0000, Stuart Henderson wrote:
> > Thought I'd take a look at a kernel profile from a machine which is
> > fairly "interesting" in terms of kernel spin. (Running -current).
> > 
> > In the attached output from running "btrace kprofile.bt" I have a
> > lot of time accounted for under syscall+0x5c4, which has _kernel_lock
> > directly next to it in the traces, and I was wondering what that is.
> > Bit confused by disasm of trap.o but my guess is that it might be
> > a lock relating to dt(4) itself, is that right?
>  
> I think this is the bit of syscall that grabs the kernel lock for any
> syscall not marked NOLOCK. In your case this is mostly fstat*, open*
> or the bits under syscall+0x5e7.

Thank you.

> Your system does a lot of parallel disk IO and that is all still using the
> KERNEL_LOCK.

oh, I should try "VFS syscalls & KERNEL_LOCK()" on this machine!