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Date: Mon, 12 Feb 2024 15:42:16 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Steven Rostedt <rostedt@...dmis.org>, LKML
<linux-kernel@...r.kernel.org>, Linux trace kernel
<linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com>, Vincent Donnefort
<vdonnefort@...gle.com>, Sven Schnelle <svens@...ux.ibm.com>, Mete Durlu
<meted@...ux.ibm.com>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix wasted memory in saved_cmdlines logic
On Thu, 2024-02-08 at 10:53 -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>
> While looking at improving the saved_cmdlines cache I found a huge amount
> of wasted memory that should be used for the cmdlines.
>
> The tracing data saves pids during the trace. At sched switch, if a trace
> occurred, it will save the comm of the task that did the trace. This is
> saved in a "cache" that maps pids to comms and exposed to user space via
> the /sys/kernel/tracing/saved_cmdlines file. Currently it only caches by
> default 128 comms.
>
> The structure that uses this creates an array to store the pids using
> PID_MAX_DEFAULT (which is usually set to 32768). This causes the structure
> to be of the size of 131104 bytes on 64 bit machines.
>
> In hex: 131104 = 0x20020, and since the kernel allocates generic memory in
> powers of two, the kernel would allocate 0x40000 or 262144 bytes to store
> this structure. That leaves 131040 bytes of wasted space.
>
> Worse, the structure points to an allocated array to store the comm names,
> which is 16 bytes times the amount of names to save (currently 128), which
> is 2048 bytes. Instead of allocating a separate array, make the structure
> end with a variable length string and use the extra space for that.
>
> This is similar to a recommendation that Linus had made about eventfs_inode names:
>
> https://lore.kernel.org/all/20240130190355.11486-5-torvalds@linux-foundation.org/
>
> Instead of allocating a separate string array to hold the saved comms,
> have the structure end with: char saved_cmdlines[]; and round up to the
> next power of two over sizeof(struct saved_cmdline_buffers) + num_cmdlines * TASK_COMM_LEN
> It will use this extra space for the saved_cmdline portion.
>
> Now, instead of saving only 128 comms by default, by using this wasted
> space at the end of the structure it can save over 8000 comms and even
> saves space by removing the need for allocating the other array.
>
> Cc: stable@...r.kernel.org
> Fixes: 939c7a4f04fcd ("tracing: Introduce saved_cmdlines_size file")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
> kernel/trace/trace.c | 73 +++++++++++++++++++++-----------------------
> 1 file changed, 34 insertions(+), 39 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 2a7c6fd934e9..0b3e60b827f7 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2320,7 +2320,7 @@ struct saved_cmdlines_buffer {
> unsigned *map_cmdline_to_pid;
> unsigned cmdline_num;
> int cmdline_idx;
> - char *saved_cmdlines;
> + char saved_cmdlines[];
> };
> static struct saved_cmdlines_buffer *savedcmd;
>
> @@ -2334,47 +2334,54 @@ static inline void set_cmdline(int idx, const char *cmdline)
> strncpy(get_saved_cmdlines(idx), cmdline, TASK_COMM_LEN);
> }
>
> -static int allocate_cmdlines_buffer(unsigned int val,
> - struct saved_cmdlines_buffer *s)
> +static void free_saved_cmdlines_buffer(struct saved_cmdlines_buffer *s)
> +{
> + int order = get_order(sizeof(*s) + s->cmdline_num * TASK_COMM_LEN);
> +
> + kfree(s->map_cmdline_to_pid);
> + free_pages((unsigned long)s, order);
> +}
> +
> +static struct saved_cmdlines_buffer *allocate_cmdlines_buffer(unsigned int val)
> {
> + struct saved_cmdlines_buffer *s;
> + struct page *page;
> + int orig_size, size;
> + int order;
> +
> + /* Figure out how much is needed to hold the given number of cmdlines */
> + orig_size = sizeof(*s) + val * TASK_COMM_LEN;
> + order = get_order(orig_size);
> + size = 1 << (order + PAGE_SHIFT);
> + page = alloc_pages(GFP_KERNEL, order);
> + if (!page)
> + return NULL;
> +
> + s = page_address(page);
> + memset(s, 0, sizeof(*s));
> +
> + /* Round up to actual allocation */
> + val = (size - sizeof(*s)) / TASK_COMM_LEN;
> + s->cmdline_num = val;
> +
> s->map_cmdline_to_pid = kmalloc_array(val,
> sizeof(*s->map_cmdline_to_pid),
> GFP_KERNEL);
> - if (!s->map_cmdline_to_pid)
> - return -ENOMEM;
> -
> - s->saved_cmdlines = kmalloc_array(TASK_COMM_LEN, val, GFP_KERNEL);
> - if (!s->saved_cmdlines) {
> - kfree(s->map_cmdline_to_pid);
> - return -ENOMEM;
> - }
>
> s->cmdline_idx = 0;
> - s->cmdline_num = val;
> memset(&s->map_pid_to_cmdline, NO_CMDLINE_MAP,
> sizeof(s->map_pid_to_cmdline));
> memset(s->map_cmdline_to_pid, NO_CMDLINE_MAP,
> val * sizeof(*s->map_cmdline_to_pid));
>
> - return 0;
> + return s;
> }
>
> static int trace_create_savedcmd(void)
> {
> - int ret;
> -
> - savedcmd = kmalloc(sizeof(*savedcmd), GFP_KERNEL);
> - if (!savedcmd)
> - return -ENOMEM;
> -
> - ret = allocate_cmdlines_buffer(SAVED_CMDLINES_DEFAULT, savedcmd);
> - if (ret < 0) {
> - kfree(savedcmd);
> - savedcmd = NULL;
> - return -ENOMEM;
> - }
> + savedcmd = allocate_cmdlines_buffer(SAVED_CMDLINES_DEFAULT);
>
> - return 0;
> + return savedcmd ? 0 : -ENOMEM;
> }
>
> int is_tracing_stopped(void)
> @@ -6056,26 +6063,14 @@ tracing_saved_cmdlines_size_read(struct file *filp, char __user *ubuf,
> return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
> }
>
> -static void free_saved_cmdlines_buffer(struct saved_cmdlines_buffer *s)
> -{
> - kfree(s->saved_cmdlines);
> - kfree(s->map_cmdline_to_pid);
> - kfree(s);
> -}
> -
> static int tracing_resize_saved_cmdlines(unsigned int val)
> {
> struct saved_cmdlines_buffer *s, *savedcmd_temp;
>
> - s = kmalloc(sizeof(*s), GFP_KERNEL);
> + s = allocate_cmdlines_buffer(val);
> if (!s)
> return -ENOMEM;
>
> - if (allocate_cmdlines_buffer(val, s) < 0) {
> - kfree(s);
> - return -ENOMEM;
> - }
> -
> preempt_disable();
> arch_spin_lock(&trace_cmdline_lock);
> savedcmd_temp = savedcmd;
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