Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof compilation time

From: Sedat Dilek
Date: Thu May 28 2020 - 12:39:46 EST

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On Thu, May 28, 2020 at 5:16 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Thu, 28 May 2020 at 04:12, Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> >
> [...]
>
> > > > >
> > > > > In general, CONFIG_KCSAN=y and the defaults for the other KCSAN
> > > > > options should be good. Depending on the size of your system, you
> > > > > could also tweak KCSAN runtime performance:
> > > > > https://lwn.net/Articles/816850/#Interacting%20with%20KCSAN%20at%20Runtime
> > > > > -- the defaults should be good for most systems though.
> > > > > Hope this helps. Any more questions, do let me know.
> > > > >
> > > >
> > > > Which "projects" and packages do I need?
> > > >
> > > > I have installed:
> > > >
> > > > # LC_ALL=C apt-get install llvm-11 clang-11 lld-11
> > > > --no-install-recommends -t llvm-toolchain -y
> > > >
> > > > # dpkg -l | grep
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261 | awk
> > > > '/^ii/ {print $1 " " $2 " " $3}' | column -t
> > > > ii clang-11
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii libclang-common-11-dev
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii libclang-cpp11
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii libclang1-11
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii libllvm11:amd64
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii lld-11
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii llvm-11
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > > ii llvm-11-runtime
> > > > 1:11~++20200527111130+65030821d4a-1~exp1~20200527091804.3261
> > > >
> > > > Is that enough?
> > >
> > > Just clang-11 (and its transitive dependencies) is enough. Unsure what
> > > your installed binary is, likely "clang-11", so if you can do "make
> > > CC=clang-11 defconfig" (and check for CONFIG_HAVE_KCSAN_COMPILER)
> > > you're good to go.
> > >
> >
> > I was able to build with clang-11 from apt.llvm.org.
> >
> > [ build-time ]
> >
> > Normally, it takes me approx. 05:00 to build with clang-10
> > (10.0.1-rc1) and Linux v5.7-rc7.
> >
> > This time start: 21:18 and stop: 03:45 means 06:27 - took 01:27 longer.
> >
> > Samsung Ultrabook 2nd generation aka Intel Sandybridge CPU with 'make -j3'.
> >
> > [ diffconfig ]
> >
> > BUILD_SALT "5.7.0-rc7-2-amd64-clang" -> "5.7.0-rc7-3-amd64-clang"
> > CLANG_VERSION 100001 -> 110000
> > +CC_HAS_ASM_INLINE y
> > +HAVE_ARCH_KCSAN y
> > +HAVE_KCSAN_COMPILER y
> > +KCSAN y
> > +KCSAN_ASSUME_PLAIN_WRITES_ATOMIC y
> > +KCSAN_DEBUG n
> > +KCSAN_DELAY_RANDOMIZE y
> > +KCSAN_EARLY_ENABLE y
> > +KCSAN_IGNORE_ATOMICS n
> > +KCSAN_INTERRUPT_WATCHER n
> > +KCSAN_NUM_WATCHPOINTS 64
> > +KCSAN_REPORT_ONCE_IN_MS 3000
> > +KCSAN_REPORT_RACE_UNKNOWN_ORIGIN y
> > +KCSAN_REPORT_VALUE_CHANGE_ONLY y
> > +KCSAN_SELFTEST y
> > +KCSAN_SKIP_WATCH 4000
> > +KCSAN_SKIP_WATCH_RANDOMIZE y
> > +KCSAN_UDELAY_INTERRUPT 20
> > +KCSAN_UDELAY_TASK 80
> >
> > I am seeing this data-races:
> >
> > root@iniza:~# LC_ALL=C dmesg -T | grep 'BUG: KCSAN: data-race'
> > [Thu May 28 03:51:53 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:52:00 2020] BUG: KCSAN: data-race in mark_page_accessed
> > / workingset_activation
> > [Thu May 28 03:52:02 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:52:08 2020] BUG: KCSAN: data-race in
> > blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> > [Thu May 28 03:52:10 2020] BUG: KCSAN: data-race in dd_has_work /
> > dd_insert_requests
> > [Thu May 28 03:52:11 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:52:13 2020] BUG: KCSAN: data-race in
> > page_counter_try_charge / page_counter_try_charge
> > [Thu May 28 03:52:15 2020] BUG: KCSAN: data-race in ep_poll_callback /
> > ep_send_events_proc
> > [Thu May 28 03:52:21 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:52:25 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:52:26 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:52:31 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:52:38 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:52:53 2020] BUG: KCSAN: data-race in dd_has_work /
> > dd_insert_requests
> > [Thu May 28 03:52:56 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:52:59 2020] BUG: KCSAN: data-race in
> > blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> > [Thu May 28 03:53:25 2020] BUG: KCSAN: data-race in
> > rwsem_spin_on_owner+0x102/0x1a0
> > [Thu May 28 03:53:25 2020] BUG: KCSAN: data-race in
> > page_counter_try_charge / page_counter_try_charge
> > [Thu May 28 03:53:39 2020] BUG: KCSAN: data-race in do_epoll_wait /
> > ep_poll_callback
> > [Thu May 28 03:53:39 2020] BUG: KCSAN: data-race in find_next_and_bit+0x30/0xd0
> > [Thu May 28 03:53:41 2020] BUG: KCSAN: data-race in dd_has_work /
> > dd_insert_requests
> > [Thu May 28 03:53:43 2020] BUG: KCSAN: data-race in do_epoll_wait /
> > ep_poll_callback
> > [Thu May 28 03:53:45 2020] BUG: KCSAN: data-race in dd_has_work /
> > dd_insert_requests
> > [Thu May 28 03:53:46 2020] BUG: KCSAN: data-race in
> > blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> > [Thu May 28 03:53:47 2020] BUG: KCSAN: data-race in
> > rwsem_spin_on_owner+0x102/0x1a0
> > [Thu May 28 03:54:02 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:54:11 2020] BUG: KCSAN: data-race in find_next_and_bit+0x30/0xd0
> > [Thu May 28 03:54:19 2020] BUG: KCSAN: data-race in
> > rwsem_spin_on_owner+0x102/0x1a0
> > [Thu May 28 03:55:00 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:56:14 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:56:50 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:56:50 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:56:52 2020] BUG: KCSAN: data-race in
> > tick_nohz_next_event / tick_nohz_stop_tick
> > [Thu May 28 03:56:58 2020] BUG: KCSAN: data-race in
> > blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> > [Thu May 28 03:57:58 2020] BUG: KCSAN: data-race in
> > blk_mq_sched_dispatch_requests / blk_mq_sched_dispatch_requests
> > [Thu May 28 03:58:00 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 03:58:07 2020] BUG: KCSAN: data-race in
> > tick_nohz_next_event / tick_nohz_stop_tick
> > [Thu May 28 03:58:44 2020] BUG: KCSAN: data-race in
> > mutex_spin_on_owner+0xe0/0x1b0
> > [Thu May 28 03:58:49 2020] BUG: KCSAN: data-race in __bitmap_subset+0x38/0xd0
> > [Thu May 28 03:59:46 2020] BUG: KCSAN: data-race in
> > tick_nohz_next_event / tick_nohz_stop_tick
> > [Thu May 28 04:00:25 2020] BUG: KCSAN: data-race in dd_has_work /
> > deadline_remove_request
> > [Thu May 28 04:00:26 2020] BUG: KCSAN: data-race in
> > tick_nohz_next_event / tick_nohz_stop_tick
> >
> > Full dmesg output and linux-config attached.
>
> Thank you for the report. There are a number of known data races. Note
> that, we do not think it's wise to rush fixes for data races,
> especially because each one requires careful analysis of what the
> appropriate response is. In the meantime, also have a look at these 2
> articles (if you haven't already), which describes the current state
> of things:
>
> 1. https://lwn.net/Articles/816850/
> 2. https://lwn.net/Articles/816854/
>

Hi Marco,

thanks for your feedback.

The first article I have read already.
That does not mean I am a KCSAN expert now.

As you say each data-race needs an individual analysis.

Just one last number:
Building again a Linux v5.7-rc7 on a clang-11-compiled and
kcsan-enabled linux-kernel took me...
one hour longer (6 instead of 5 hours, start: 12:14 and stop: 18:15)

Regards,
- Sedat -

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In reply to: Marco Elver: "Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof compilation time"
Next in thread: tip-bot2 for Marco Elver: "[tip: locking/kcsan] compiler_types.h: Optimize __unqual_scalar_typeof compilation time"
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