linux-kernel - Re: [PATCH] mm/khugepaged: add tracepoint to collapse

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Date:   Mon, 24 Oct 2022 11:16:21 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Gautam Menghani <gautammenghani201@...il.com>
Cc:     akpm@...ux-foundation.org, mhiramat@...nel.org, zokeefe@...gle.com,
        shy828301@...il.com, vbabka@...e.cz, david@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/khugepaged: add tracepoint to collapse_file()

On Mon, 24 Oct 2022 20:39:22 +0530
Gautam Menghani <gautammenghani201@...il.com> wrote:

> +TRACE_EVENT(mm_khugepaged_collapse_file,
> +	TP_PROTO(struct mm_struct *mm, struct page *hpage,
> +			unsigned long addr, const char *filename, int nr,
> +			int result),
> +	TP_ARGS(mm, hpage, addr, filename, nr, result),
> +	TP_STRUCT__entry(
> +		__field(struct mm_struct *, mm)
> +		__field(unsigned long, hpfn)
> +		__field(unsigned long, addr)
> +		__string(filename, filename)
> +		__field(int, nr)
> +		__field(int, result)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->mm = mm;
> +		__entry->hpfn = hpage ? page_to_pfn(hpage) : -1;
> +		__entry->addr = addr;
> +		__assign_str(filename, filename);
> +		__entry->nr = nr;
> +		__entry->result = result;
> +	),
> +
> +	TP_printk("mm=%p, hpage_pfn=0x%lx, addr=%ld, filename=%s, nr=%d, result=%s",
> +		__entry->mm,
> +		__entry->hpfn,
> +		__entry->addr,
> +		__get_str(filename),
> +		__entry->nr,
> +		__print_symbolic(__entry->result, SCAN_STATUS))
> +);
> +
>  #endif /* __HUGE_MEMORY_H */
>  #include <trace/define_trace.h>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4734315f7940..14db90e2f2ec 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2059,7 +2059,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  		mem_cgroup_uncharge(page_folio(hpage));
>  		put_page(hpage);
>  	}
> -	/* TODO: tracepoints */
> +
> +	trace_mm_khugepaged_collapse_file(mm, hpage, addr, file->f_path.dentry->d_iname,

I try to keep dereferences out of the calling path as much as possible
(adds to I$ at the call site).

Could you just pass in file, and then have:

	__string(filename, file->f_path.dentry->d_iname)

	[..]

	__assign_string(filename, file->f_path.dentry->d_iname);


If you are paranoid, you can have the above also be:

			file ? file->f_path.dentry ? file->f_path.dentry->d_iname : "(null)" : "(null)")


-- Steve


> +				      nr, result);
>  	return result;