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Message-ID: <20221114103350.4b8a2786@gandalf.local.home>
Date: Mon, 14 Nov 2022 10:33:50 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Gautam Menghani <gautammenghani201@...il.com>
Cc: akpm@...ux-foundation.org, mhiramat@...nel.org,
shy828301@...il.com, zokeefe@...gle.com, david@...hat.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/khugepaged: add tracepoint to collapse_file()
On Wed, 26 Oct 2022 10:52:18 +0530
Gautam Menghani <gautammenghani201@...il.com> wrote:
Ideally we want the trace event structure to be as packed as possible to
not waste space on the ring buffer.
> + TP_ARGS(mm, hpage, index, is_shmem, addr, file, nr, result),
> + TP_STRUCT__entry(
> + __field(struct mm_struct *, mm) 4 / 8 bytes (depending on 32 bit or 64 bit arch)
> + __field(unsigned long, hpfn) 4 / 8 bytes
The two above is fine.
> + __field(pgoff_t, index) 4 / 8 bytes
> + __field(bool, is_shmem) 4 bytes (or less)
> + __field(unsigned long, addr) 4 / 8 bytes
> + __string(filename, file->f_path.dentry->d_iname) 4 bytes
> + __field(int, nr) 4 bytes
> + __field(int, result) 4 bytes
For best packing, it's best to keep long / pointers together, and ints and
bools together (or paired).
On 64 bit archs, there is likely to be a 4 byte hole between is_shmem and
addr.
> + ),
> +
Better to have it be:
TP_STRUCT__entry(
__field(struct mm_struct *, mm)
__field(unsigned long, hpfn)
__field(pgoff_t, index)
__field(unsigned long, addr)
__field(bool, is_shmem)
__string(filename, file->f_path.dentry->d_iname)
__field(int, nr)
__field(int, result)
),
Where I swapped is_shmem and addr.
-- Steve
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