85 lines
4.0 KiB
Diff
85 lines
4.0 KiB
Diff
commit e5ba7aa2af74fd22ebbd5c4a6624edcf983863de
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Author: Michal Schmidt <mschmidt@redhat.com>
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Date: Fri Aug 4 16:53:01 2023 +0200
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gps/gps.py.in: no busy-waiting when reading from gpsd socket
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ubxtool keeps one CPU 100% busy while it waits for data to read from the
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gpsd socket. Running it under strace showed that it calls select() with
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zero timeout in a loop:
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...
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11:02:34.049629 pselect6(4, [3], [], [], {tv_sec=0, tv_nsec=0}, NULL) = 0 (Timeout)
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11:02:34.049649 pselect6(4, [3], [], [], {tv_sec=0, tv_nsec=0}, NULL) = 0 (Timeout)
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11:02:34.049670 pselect6(4, [3], [], [], {tv_sec=0, tv_nsec=0}, NULL) = 0 (Timeout)
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...
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The busy waiting can be eliminated by passing the actual timeout value
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to select(). In the reading loop in gps.py, the remaining time can be
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easily calculated and passed as the argument to the self.ser.waiting()
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function (which is basically a select() wrapper).
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Fixing this problem exposed a bug in how the received bytes are decoded.
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decode_func may not consume all input at once. Consumable input may be
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left in self.out until decode_func returns zero, indicating that it
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could not process any more input. So decode_func must be called in a
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loop each time a buffer is received from the socket. The busy waiting
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was hiding this issue, because decode_func was being called all the
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time.
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The "elif self.input_is_device:" branch probably needs similar
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treatment, but I am testing only the gpsd usecase.
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diff --git a/gps/gps.py.in b/gps/gps.py.in
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index 623a750a0..14d7707ab 100644
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--- a/gps/gps.py.in
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+++ b/gps/gps.py.in
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@@ -384,10 +384,11 @@ class gps_io(object):
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if self.gpsd_host is not None:
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# gpsd input
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start = monotonic()
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- while (monotonic() - start) < input_wait:
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+ remaining_time = input_wait
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+ while remaining_time > 0:
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# First priority is to be sure the input buffer is read.
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# This is to prevent input buffer overuns
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- if 0 < self.ser.waiting():
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+ if 0 < self.ser.waiting(remaining_time):
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# We have serial input waiting, get it
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# No timeout possible
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# RTCM3 JSON can be over 4.4k long, so go big
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@@ -397,17 +398,22 @@ class gps_io(object):
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raw_fd.write(polybytes(new_out))
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self.out += new_out
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- consumed = decode_func(self.out)
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- # TODO: the decoder shall return a some current
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- # statement_identifier # to fill last_statement_identifier
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- last_statement_identifier = None
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- #
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- self.out = self.out[consumed:]
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- if ((expect_statement_identifier and
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- (expect_statement_identifier ==
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- last_statement_identifier))):
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- # Got what we were waiting for. Done?
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- ret_code = 0
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+ while True:
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+ consumed = decode_func(self.out)
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+ if consumed == 0:
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+ break
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+ # TODO: the decoder shall return a some current
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+ # statement_identifier # to fill last_statement_identifier
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+ last_statement_identifier = None
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+ #
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+ self.out = self.out[consumed:]
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+ if ((expect_statement_identifier and
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+ (expect_statement_identifier ==
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+ last_statement_identifier))):
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+ # Got what we were waiting for. Done?
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+ ret_code = 0
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+
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+ remaining_time = start + input_wait - monotonic()
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elif self.input_is_device:
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# input is a serial device
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