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Subject: FW: Linux  preempt policy serial port communication
Date: Tue, 29 Jun 2010 17:06:15 +0800
Message-ID: <9BC2BBFFF6E8F04094DB58B805758CE406C02B63@SHAMLVEM01.e2k.ad.ge.com>
Thread-Topic: Linux  preempt policy serial port communication
Thread-Index: AcsXJRyaeRhKhx7WTBSzPwZuGIjixgANhFNgAAPH76A=
From: "Zhang, Yongchao (GE Healthcare)" <Yongchao.Zhang@ge.com>
To: <linux-kernel@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 6507
Lines: 175

Hi, 
   We have some problems about serial port. We need your expertise. The
detail is like this:
 
1.Our Goal:
   We want to make test on serial port, and want to know whether the
serial port communication is still OK  under heavy system load.
 
2.Environment:
     1). Two computer,one is as server,another is as client.
         The "Server" sends data to serial port. We use a serial port
tool run on WINXP as server.
         The "Client" reads the data from serial port,and run on Linux.
 
3.The way access serial port for "client" program:
     During test procedure, we use  two kinds of way to "read" data from
serial port. One way is "select +none block read" ,another is  "block
read".
     The code slice  is as the follwoing:
         (1).select+none block read. like this: 
                   ......
                   while(num_of_bytes < len)
                   {
                           //wait till notified.
                           while((select(m_fileDescriptor+1,&m_readSet,
NULL, NULL, NULL)) == -1 && errno == EINTR)
                           {
                                  usleep(1000);
                           }

                           //read data
                           if((readBytes = ::read(m_fileDescriptor,
(buff+num_of_bytes), (len-num_of_bytes))) == -1)
                           {
                                  close();
 
printf("CS_ERR_READ_SERIAL_PORT_FAIL");
                                  exit(0);
                           }
                           else
                           {   
                                  num_of_bytes += readBytes;
                           }
                 }
                 ......
              
        (2).block read
                first set paramer c_cc as this during init. 
                    port_settings.c_cc[VMIN ]  = 1 ;
                    port_settings.c_cc[VTIME] = 0 ; 
                second,read directly
                    readBytes = ::read(m_fileDescriptor,
(buff+num_of_bytes), (len-num_of_bytes))) == -1
   
4. The heavy load simulation program.
          Here we write a program to simulate heavy CPU load in linux
system. This "heavy load program" create one thread and promote thread
prority to 99, which is the top most in system. This thread did nothing
but empty loop and never release CPU resource actively.The code slice as
following:
 
           //create this thread
           ....
           pthread_create(&m_hThread[1],NULL,fifo_99_1  ,(void*)&trans)
           ....
 
           //thread proc
           void* fifo_99(void *para)
           {
               struct sched_param param;

               //adjust prority to FIFO 99.
               pthread_t thread_t     = pthread_self();
               param.sched_priority = 99;
               pthread_setschedparam(thread_t,SCHED_FIFO,&param);
  
               //only cost CPU resource
               while(1)
               {
                   for (int z = 0; z < 10000; z++)
                   {
                   }
               }
           }
      
5. Test procedure
    (1).
         Procedure:
            Run serial port client program,the program is just waitting
serial port data.  The "client" program is run with normal priority.
            Run 99 FIFO emluator,so the CPU cost is 100% almost now.
            Run serial tool in WindowXP,and send data to client,total
send several groups of data.
 
         Test result: 
            Find the "client" program could not recevie the data during
FIFO 99 emluator running. But if break the emluator , the "client"
program could read out all the data
  immediately. We get know that it is "select" statement that always
blocked without waken up if FIFO 99 running . So it seems the system
could not notify the "serial port data received signal" to "select"
statement in time.
 
         Analysis &  Question:
            Why FIFO 99 could affect on serial port read? Could you
please point out the detail code position(maybe in serial port driver)?
And how to avoid this to let "serial port reading" could not be
disturbed by any other thread or process even if FIFO 99?
 
   (2).
        Procedure
           The same procedure with (1). But this time we also promote
the "client" program to FIFO 99.
        Test result:
           The same with (1).
        Analysis & Question:
           Once we doube that the thread priority of the "client"
program is too low to get CPU resource, so we promote "client" program
also to FIFO 99.
           But also could not read out the data until heavy load is
over. Maybe even high priority for "client" program have no effect on
this.
 
   (3).
         Procedure:
            The same with (1). But this time change FIFO 99 to FIFO 98,
and also "client" program is run with normal priority.
         Test result:     
            The "client" program could read the data in time,all is OK.
         Analysis & Question:
            FIFO 99,98 are both high prority in system. But 98 could not
affect on the serial port,99 could. FIFO 99,98 are both high priority,
why is it so much difference for serial port reading?
 
   (4). 
         Procedure:
            The same with (3). But this time we run two heavy load
programs i.e. two FIFO 98 in linux system.
         Test result:
            There is much difference with (1) and (3).  We will find
"client" program could receive the data but have some delay(about 2~4
seconds delay)
         Analysis & Question:
            Two FIFO98 are also heavy load to system,but compared with
FIFO 99,FIFO 98 is not so rigorous. So system maybe give "some running
slice" to notify the "select" statement though could not be in time but
still have opportunity to notify. Right?
 
 
   From our tests, we want to consult with you following questions:
   
   1. Why FIFO 99 could affect on serial port read? Could you please
point out the detail code position(maybe in serial port driver)?  And
how to avoid this to let "serial port reading" could not be disturbed by
any other thread or process even if FIFO 99?
 
   2. From procedure (2),To FIFO 99,98,why is there so much difference
for serial port reading?
 
 
Thanks!
 
Best Regards!
 
-------------------------
Philip
2010.06.29
-------------------------
 
 
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