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Date: Fri, 22 Jan 2021 17:07:50 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/3] tracing: Merge irqflags + preempt counter.
On Wed, 13 Jan 2021 00:00:55 +0100
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> The state of the interrupts (irqflags) and the preemption counter are
> both passed down to tracing_generic_entry_update(). Only one bit of
> irqflags is actually required: The on/off state. The complete 32bit
> of the preemption counter isn't needed. Just whether of the upper bits
> (softirq, hardirq and NMI) are set and the preemption depth is needed.
>
> The irqflags and the preemption counter could be evaluated early and the
> information stored in an integer `trace_ctx'.
> tracing_generic_entry_update() would use the upper bits as the
> TRACE_FLAG_* and the lower 8bit as the disabled-preemption depth
> (considering that one must be substracted from the counter in one
> special cases).
>
> The actual preemption value is not used except for the tracing record.
> The `irqflags' variable is mostly used only for the tracing record. An
> exception here is for instance wakeup_tracer_call() or
> probe_wakeup_sched_switch() which explicilty disable interrupts and use
> that `irqflags' to save (and restore) the IRQ state and to record the
> state.
>
> Struct trace_event_buffer has also the `pc' and flags' members which can
> be replaced with `trace_ctx' since their actual value is not used
> outside of trace recording.
>
> This will reduce tracing_generic_entry_update() to simply assign values
> to struct trace_entry. The evaluation of the TRACE_FLAG_* bits is moved
> to _tracing_gen_ctx_flags() which replaces preempt_count() and
> local_save_flags() invocations.
>
> As an example, ftrace_syscall_enter() may invoke:
> - trace_buffer_lock_reserve() -> … -> tracing_generic_entry_update()
> - event_trigger_unlock_commit()
> -> ftrace_trace_stack() -> … -> tracing_generic_entry_update()
> -> ftrace_trace_userstack() -> … -> tracing_generic_entry_update()
>
> In this case the TRACE_FLAG_* bits were evaluated three times. By using
> the `trace_ctx' they are evaluated once and assigned three times.
>
> A build with all tracers enabled on x86-64 with and without the patch:
>
> text data bss dec hex filename
> 21970669 17084168 7639260 46694097 2c87ed1 vmlinux.old
> 21970293 17084168 7639260 46693721 2c87d59 vmlinux.new
>
> text shrank by 379 bytes, data remained constant.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> include/linux/trace_events.h | 25 +++-
> kernel/trace/blktrace.c | 17 +--
> kernel/trace/trace.c | 208 ++++++++++++++-------------
> kernel/trace/trace.h | 38 +++--
> kernel/trace/trace_branch.c | 6 +-
> kernel/trace/trace_event_perf.c | 5 +-
> kernel/trace/trace_events.c | 18 +--
> kernel/trace/trace_events_inject.c | 6 +-
> kernel/trace/trace_functions.c | 28 ++--
> kernel/trace/trace_functions_graph.c | 32 ++---
> kernel/trace/trace_hwlat.c | 7 +-
> kernel/trace/trace_irqsoff.c | 86 +++++------
> kernel/trace/trace_kprobe.c | 10 +-
> kernel/trace/trace_mmiotrace.c | 14 +-
> kernel/trace/trace_sched_wakeup.c | 71 +++++----
> kernel/trace/trace_syscalls.c | 20 ++-
> kernel/trace/trace_uprobe.c | 4 +-
> 17 files changed, 287 insertions(+), 308 deletions(-)
>
> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index d321fe5ad1a14..aa9123878e126 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -148,17 +148,29 @@ enum print_line_t {
>
> enum print_line_t trace_handle_return(struct trace_seq *s);
>
> -void tracing_generic_entry_update(struct trace_entry *entry,
> - unsigned short type,
> - unsigned long flags,
> - int pc);
> +static inline void tracing_generic_entry_update(struct trace_entry *entry,
> + unsigned short type,
> + unsigned int trace_ctx)
> +{
> + struct task_struct *tsk = current;
> +
> + entry->preempt_count = trace_ctx & 0xff;
> + entry->pid = (tsk) ? tsk->pid : 0;
> + entry->type = type;
> + entry->flags = trace_ctx >> 16;
> +}
> +
> +unsigned int _tracing_gen_ctx_flags(unsigned long irqflags);
> +unsigned int tracing_gen_ctx_flags(void);
> +unsigned int tracing_gen_ctx_flags_dect(void);
Since I'm now trying to avoid underscore functions when possible, the above
should be:
_tracing_gen_ctx_flags() --> tracing_gen_ctx_flags()
tracing_gen_ctx_flags() --> tracing_gen_ctx()
tracing_gen_ctx_flags_dect --> tracing_gen_ctx_dec()
Note, "_dec()" should be used instead of "_dect()" as "_dec" is commonly
used around the kernel, and I had really no idea what "_dect" was, until I
looked at the two functions.
And looking at the implementation, I wonder if we should make this into
static inlines in the header as follows:
unsigned int tracing_gen_ctx_irq_test(int irqs_status);
#ifdef CONFIG_TRACE_IRQFLAGS_SUPPORT
static inline unsigned int tracing_gen_ctx_flags(unsigned long irqflags)
{
unsigned int irq_status = irqs_disabled(irqflags) ?
TRACE_FLAG_IRQS_OFF : 0;
return tracing_gen_ctx_irq_test(irq_status);
}
static inline unsigned int tracing_gen_ctx(void)
{
unsigned long irqflags;
local_save_flags(irqflags);
return tracing_gen_ctx_flags(irqflags);
}
#else
static inline unsigned int tracing_gen_ctx_flags(unsigned long irqflags)
{
return tracing_gen_ctx_irq_test(TRACE_FLAG_IRQS_NOSUPPORT);
}
static inline unsigned int tracing_gen_ctx(void)
{
return tracing_gen_ctx_irq_test(TRACE_FLAG_IRQS_NOSUPPORT);
}
#endif
static inline unsigned int tracing_gen_ctx_dec(void)
{
unsigned int trace_ctx;
trace_ctx = tracing_gen_ctx();
/*** Comment about buffer_reserve here ***/
if (IS_ENABLED(CONFIG_PREEMPTION))
trace_ctx--;
return trace_ctx;
}
Then all you need to do is implement the tracing_gen_ctx_irq_test() without
adding and #ifdefs in it, and just or in the "irq_status" to trace_flags,
without any conditionals.
-- Steve
> +
> struct trace_event_file;
>
> struct ring_buffer_event *
> trace_event_buffer_lock_reserve(struct trace_buffer **current_buffer,
> struct trace_event_file *trace_file,
> int type, unsigned long len,
> - unsigned long flags, int pc);
> + unsigned int trace_ctx);
>
> #define TRACE_RECORD_CMDLINE BIT(0)
> #define TRACE_RECORD_TGID BIT(1)
> @@ -232,8 +244,7 @@ struct trace_event_buffer {
> struct ring_buffer_event *event;
> struct trace_event_file *trace_file;
> void *entry;
> - unsigned long flags;
> - int pc;
> + unsigned int trace_ctx;
> struct pt_regs *regs;
> };
>
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