Re: [PATCH v7 09/13] user_events: Optimize writing events by only copying data once

Masami Hiramatsu
2021-12-10T14:51:18Z

Hi Beau,

On Thu, 9 Dec 2021 14:32:06 -0800
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> Pass iterator through to probes to allow copying data directly to the
> probe buffers instead of taking multiple copies. Enables eBPF user and
> raw iterator types out to programs for no-copy scenarios.
>
> Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> ---
> kernel/trace/trace_events_user.c | 102 ++++++++++++++++++++++---------
> 1 file changed, 74 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 807db0af74fb..1d29f6ec907d 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -41,6 +41,10 @@
> #define MAX_FIELD_ARRAY_SIZE (2 * PAGE_SIZE)
> #define MAX_FIELD_ARG_NAME 256
>
> +#define MAX_BPF_COPY_SIZE PAGE_SIZE
> +#define MAX_STACK_BPF_DATA 512
> +#define copy_nofault copy_from_iter_nocache
> +
> static char *register_page_data;
>
> static DEFINE_MUTEX(reg_mutex);
> @@ -78,8 +82,7 @@ struct user_event_refs {
> struct user_event *events[];
> };
>
> -typedef void (*user_event_func_t) (struct user_event *user,
> - void *data, u32 datalen,
> +typedef void (*user_event_func_t) (struct user_event *user, struct iov_iter *i,
> void *tpdata);
>
> static int user_event_parse(char *name, char *args, char *flags,
> @@ -515,7 +518,7 @@ static struct user_event *find_user_event(char *name, u32 *outkey)
> /*
> * Writes the user supplied payload out to a trace file.
> */
> -static void user_event_ftrace(struct user_event *user, void *data, u32 datalen,
> +static void user_event_ftrace(struct user_event *user, struct iov_iter *i,
> void *tpdata)
> {
> struct trace_event_file *file;
> @@ -531,41 +534,85 @@ static void user_event_ftrace(struct user_event *user, void *data, u32 datalen,
>
> /* Allocates and fills trace_entry, + 1 of this is data payload */
> entry = trace_event_buffer_reserve(&event_buffer, file,
> - sizeof(*entry) + datalen);
> + sizeof(*entry) + i->count);
>
> if (unlikely(!entry))
> return;
>
> - memcpy(entry + 1, data, datalen);
> + if (unlikely(!copy_nofault(entry + 1, i->count, i))) {
> + __trace_event_discard_commit(event_buffer.buffer,
> + event_buffer.event);
> + return;
> + }
>
> trace_event_buffer_commit(&event_buffer);
> }
>
> #ifdef CONFIG_PERF_EVENTS
> +static void user_event_bpf(struct user_event *user, struct iov_iter *i)
> +{
> + struct user_bpf_context context;
> + struct user_bpf_iter bpf_i;
> + char fast_data[MAX_STACK_BPF_DATA];
> + void *temp = NULL;
> +
> + if ((user->flags & FLAG_BPF_ITER) && iter_is_iovec(i)) {
> + /* Raw iterator */
> + context.data_type = USER_BPF_DATA_ITER;
> + context.data_len = i->count;
> + context.iter = &bpf_i;
> +
> + bpf_i.iov_offset = i->iov_offset;
> + bpf_i.iov = i->iov;
> + bpf_i.nr_segs = i->nr_segs;
> + } else if (i->nr_segs == 1 && iter_is_iovec(i)) {
> + /* Single buffer from user */
> + context.data_type = USER_BPF_DATA_USER;
> + context.data_len = i->count;
> + context.udata = i->iov->iov_base + i->iov_offset;
> + } else {
> + /* Multi buffer from user */
> + struct iov_iter copy = *i;
> + size_t copy_size = min(i->count, (size_t)MAX_BPF_COPY_SIZE);
> +
> + context.data_type = USER_BPF_DATA_KERNEL;
> + context.kdata = fast_data;
> +
> + if (unlikely(copy_size > sizeof(fast_data))) {
> + temp = kmalloc(copy_size, GFP_NOWAIT);
> +
> + if (temp)
> + context.kdata = temp;
> + else
> + copy_size = sizeof(fast_data);
> + }
> +
> + context.data_len = copy_nofault(context.kdata,
> + copy_size, &copy);
> + }
> +
> + trace_call_bpf(&user->call, &context);
> +
> + kfree(temp);
> +}
> +
> /*
> * Writes the user supplied payload out to perf ring buffer or eBPF program.
> */
> -static void user_event_perf(struct user_event *user, void *data, u32 datalen,
> +static void user_event_perf(struct user_event *user, struct iov_iter *i,
> void *tpdata)
> {
> struct hlist_head *perf_head;
>
> - if (bpf_prog_array_valid(&user->call)) {
> - struct user_bpf_context context = {0};
> -
> - context.data_len = datalen;
> - context.data_type = USER_BPF_DATA_KERNEL;
> - context.kdata = data;
> -
> - trace_call_bpf(&user->call, &context);
> - }
> + if (bpf_prog_array_valid(&user->call))
> + user_event_bpf(user, i);
>
> perf_head = this_cpu_ptr(user->call.perf_events);
>
> if (perf_head && !hlist_empty(perf_head)) {
> struct trace_entry *perf_entry;
> struct pt_regs *regs;
> - size_t size = sizeof(*perf_entry) + datalen;
> + size_t size = sizeof(*perf_entry) + i->count;
> int context;
>
> perf_entry = perf_trace_buf_alloc(ALIGN(size, 8),
> @@ -576,7 +623,10 @@ static void user_event_perf(struct user_event *user, void *data, u32 datalen,
>
> perf_fetch_caller_regs(regs);
>
> - memcpy(perf_entry + 1, data, datalen);
> + if (unlikely(!copy_nofault(perf_entry + 1, i->count, i))) {
> + perf_swevent_put_recursion_context(context);
> + return;
> + }
>
> perf_trace_buf_submit(perf_entry, size, context,
> user->call.event.type, 1, regs,
> @@ -1009,32 +1059,28 @@ static ssize_t user_events_write_core(struct file *file, struct iov_iter *i)
> if (likely(atomic_read(&tp->key.enabled) > 0)) {
> struct tracepoint_func *probe_func_ptr;
> user_event_func_t probe_func;
> + struct iov_iter copy;
> void *tpdata;
> - void *kdata;
> - u32 datalen;
>
> - kdata = kmalloc(i->count, GFP_KERNEL);
> -
> - if (unlikely(!kdata))
> - return -ENOMEM;
> -
> - datalen = copy_from_iter(kdata, i->count, i);
> + if (unlikely(iov_iter_fault_in_readable(i, i->count)))
> + return -EFAULT;
>
> rcu_read_lock_sched();
> + pagefault_disable();

Since the pagefault_disable() may have unexpected side effect,
I think it should be used really limited area, e.g. around
actual memory access function.
Can we move this around the copy_nofault()?

Thank you,
>
> probe_func_ptr = rcu_dereference_sched(tp->funcs);
>
> if (probe_func_ptr) {
> do {
> + copy = *i;
> probe_func = probe_func_ptr->func;
> tpdata = probe_func_ptr->data;
> - probe_func(user, kdata, datalen, tpdata);
> + probe_func(user, &copy, tpdata);
> } while ((++probe_func_ptr)->func);
> }
>
> + pagefault_enable();
> rcu_read_unlock_sched();
> -
> - kfree(kdata);
> }
>
> return ret;
> --
> 2.17.1
>

--
Masami Hiramatsu <mhiramat@kernel.org>