Re: [RFC PATCH v1 18/57] trace: Remove PAGE_SIZE compile-time constant assumption

Ryan Roberts
2024-10-15T11:15:11Z

On 14/10/2024 17:46, Steven Rostedt wrote:
> On Mon, 14 Oct 2024 11:58:25 +0100
> Ryan Roberts <ryan.roberts@arm.com> wrote:
>
>> To prepare for supporting boot-time page size selection, refactor code
>> to remove assumptions about PAGE_SIZE being compile-time constant. Code
>> intended to be equivalent when compile-time page size is active.
>>
>> Convert BUILD_BUG_ON() BUG_ON() since the argument depends on PAGE_SIZE
>> and its not trivial to test against a page size limit.
>>
>> Redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is always
>> sized at 32K for 64-bit and 16K for 32-bit. It was previously defined in
>> terms of PAGE_SIZE (and worked out at the quoted sizes for a 4K page
>> size). But for 64K pages, the size expanded to 512K. Given the ftrace
>> stacks should be invariant to page size, this seemed like a waste. As a
>> side effect, it removes the PAGE_SIZE compile-time constant assumption
>> from this code.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>
>> ***NOTE***
>> Any confused maintainers may want to read the cover note here for context:
>> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>>
>> kernel/trace/fgraph.c | 2 +-
>> kernel/trace/trace.c | 2 +-
>> 2 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
>> index d7d4fb403f6f0..47aa5c8d8090e 100644
>> --- a/kernel/trace/fgraph.c
>> +++ b/kernel/trace/fgraph.c
>> @@ -534,7 +534,7 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
>> if (!current->ret_stack)
>> return -EBUSY;
>>
>> - BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long));
>> + BUG_ON(SHADOW_STACK_SIZE % sizeof(long));
>
> Absolutely not!
>
> BUG_ON() is in no way a substitution of any BUILD_BUG_ON(). BUILD_BUG_ON()
> is a non intrusive way to see if something isn't lined up correctly, and
> can fix it before you execute any code. BUG_ON() is the most intrusive way
> to say something is wrong and you crash the system.

Yep, totally agree. I'm afraid this was me being lazy, and there are a couple of
other instances where I have done this in other patches that I'll need to fix.
Most of the time, I've been able to keep BUILD_BUG_ON() and simply compare
against a page size limit.

Looking at this again, perhaps the better solution is to define
SHADOW_STACK_SIZE as PAGE_SIZE_MIN? Then it remains a compile-time constant. Is
there any need for SHADOW_STACK_SIZE to increase with page size?

>
> Not to mention, when function graph tracing is enabled, this gets triggered
> for *every* function call! So I do not want any runtime test done. Every
> nanosecond counts in this code path.
>
> If anything, this needs to be moved to initialization and checked once, if
> it fails, gives a WARN_ON() and disables function graph tracing.

I'm hoping my suggestion above to decouple SHADOW_STACK_SIZE from PAGE_SIZE is
acceptable and simpler? If not, happy to do as you suggest here.

Thanks,
Ryan

>
> -- Steve
>
>
>>
>> /* Set val to "reserved" with the delta to the new fgraph frame */
>> val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET;
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index c3b2c7dfadef1..0f2ec3d30579f 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -2887,7 +2887,7 @@ trace_function(struct trace_array *tr, unsigned long ip, unsigned long
>> /* Allow 4 levels of nesting: normal, softirq, irq, NMI */
>> #define FTRACE_KSTACK_NESTING 4
>>
>> -#define FTRACE_KSTACK_ENTRIES (PAGE_SIZE / FTRACE_KSTACK_NESTING)
>> +#define FTRACE_KSTACK_ENTRIES (SZ_4K / FTRACE_KSTACK_NESTING)
>>
>> struct ftrace_stack {
>> unsigned long calls[FTRACE_KSTACK_ENTRIES];
>